I agree. The best use of a companion is to be a complementary PoV as the party travels through the narrative. (I haven't played the sequel to Planescape, yet, but I like the sound of those characters.) The difficulty is that humans are writing what they think these incomprehensibly different characters will be thinking, which is literally ineffable. But a laconic character who acts rather than talks would be eminently possible.
I remember when I first read the P&P background to the Githyanki (AD&D2 supplemental book, IIRC) and it immediately whetted my appetite.
Yeah, the exploration of "the other" by proxy, in this case an illithid travelling with a group rather than living in a polis with its mates. It could also work for angelic characters, too, of course. The set-up is important; why is this over-powered character gimped and wandering the universe with a bunch of misfits. Usually the offspring following some ritualistic right-of-passage would suffice to explain it.

As luck would have it, I actually just ran into that "Beholder" again on my SoA playthrough. ;) For the sake of nitpicking though, I should point out that that particular Beholder is actually a "Spectator", a sub-race of Beholder-kin that's not prone to the same kind of evil megalomania that's common among the beholder races. They are generally Lawful Neutral, hence their typical encounter in dungeons serving as guardians for some sort of treasure. Bioware didn't bother giving the Spectator his own unique model, however; he got stuck with the standard Beholder model. XD

It's not a sequel literally. TToN is more like a spiritual heir. It has a different setting, but gameplay mechanics are quite similar. You'll encounter lots of text that resembles reading books. TToN is not as good as PT, but it's still worth at least one plathrough. :)

Personally I like the party composition that includes characters with different alignments, so yes, such a companion can be a nice addition to good party as well. I have high hopes for interesting companions in BG2.
I forgot that he's present in SoA as well. You'll meet him in TB once again and it's hilarious like the first encounter. :) As for the sub-race, good point. This simplifies the possible problems with such a companion. If they decide to include a Spectator companion, it'll be easier to explain why such a creature is traveling with our party.

Well, a "sequentially-released extended examination of the canon with alternate characters" seemed a bit clumsy for a simple reply, when "sequel" includes the concept that you identified … :)

I'm a little conflicted about the whole "alignment" concept. Whilst it has some merit, I find it pretty useless in some common social situations.

To call that lawful-evil or even chaotic-evil is probably okay for gaming, but then what about Dirty Harry? Is he lawful-neutral? He ain't lawful-good, perhaps chaotic-good but that doesn't account for his internal moral code: he seeks "justice". Judge Dredd? He channels his anger to punish the wicked, sure, but he also enjoys hurting the bad guys with justified pain, which doesn't really fit the lawful-good archetype (for my interpretation, in any case).*

Almost everybody thinks they are rational and principled. All people have blind spots, however, where cognitive dissonance is hidden by the mind trying to shield itself from what it doesn't want to believe about itself (which includes inconsistencies that break their own "universal" laws of behaviour). I suppose the best attempt yet has followed on from Freud and Jung's early C20th work, and especially Katharine Briggs and her daughter, Isabel Myers, who propounded the theory of mental templates for experiential organization; and, thus the common personality axes.

Also, evil has a distinctly theological and moral flavour. (There are evil individuals who "enjoy" harming others, but they are rare. More common is selfishness.) Therefore, it might be better to identify people as sociopathic, rather than evil, for instance, since a sociopath is very charming in order to gain the confidence of others to achieve their goal, but only fakes empathy to achieve it. (Literally; a sociopath has no empathy, but can imitate it very well —— through continual practice —— as a means to an end. And everyone is a means to the sociopath's ends.) Sociopaths are not necessarily evil, though. The Spectator would be a sociopath, and so, too, would an illithid (who would have no compunction in suddenly eating a human's brain, should they offer no feasible utility nor threat).

So I agree that different alignments are an interesting mix. :)

On-topic:
This would be interesting to explore in the narrative as it rarely is (cinema and literature non obstat) and would be an interesting twist for a romance. The urge to be a mindless servant of the object of desire is a common one; it is equally commonly soured into an abusive relationship (how can the loved one respect the lover who has no self-respect?) and only rarely results in a reciprocal exchange of devotion. So one partner invariably exploits the ardour of the other, for immediate or longer-term gain. Again, this does not necessarily imply or connote evil, merely a momentary (if disinterested) exploitation (starting from the innocent acceptance of the terms offered). "He wanted me to buy the big diamond with all his worldly assets!" she said, just before losing interest and dating someone else.

So often the "tart with a heart" trope is the standard romance option for a male character (think Viconia). What about a romance option that varies depending on the dialog choices, so that someone who behaves like a doormat is treated like one, but the one who asserts a self-respect and offers similar is rewarded with a healthy relationship. Perhaps it might even help the player to develop interpersonal skills, as an example of Cognitive Behaviour Therapy. And it wouldn't be too difficult to adapt the character to be bisexual, so that a female PC would be able to romance her too. Or a set-up question, earlier, might determine the gender preferences for the sex and orientation of the romantic counterpart, so that a gay or lesbian (or intersexed or transexual) could be romanced.

________
* Part of this has concerned philosophy since before Socrates, too, since it is the problem of extracting general traits from specific instances. This is due to the slippery nature of symbolic representation, necessary but not sufficient to describe morality. The focus on the inner moral landscape cannot be divorced from the utilitarian empirical impact; morality depends, by definition, on the motivation for an act, whereas the inherent goodness of an act must be judged afterwards, ex postfacto, based on its effect/s. So it is both subjective (motivation) and objectively weighted (through the result as deemed by succeeding analysis).

Dirty Harry seeks justice indeed, but it's not that kind of justice which lawful characters seek. Lawful Neutral is for characters which have faith in strong government which executes law. Dirty Harry is above the rules of government forces. He wants to do good no matter the consequences, so Chaotic Good is more adequate for him. As for Judge Dredd, he executes rules provided by the government, so it's Lawful Neutral for him. A good example of another Lawful Neutral character is Vhailor from PT.

Sociopaths have their alignment in D&D. It's Chaotic Neutral. It fits the description you provided. Except those sociopaths which enjoy harming others. These are clearly Chaotic Evil. But again not all Chaotic Evil characters enjoy harming others. When talking about alignments, we have to remember that it's all contractual. Main purpose of it is to provide fun not to precisely describe reality. Are you a psychologist perhaps? :)

You make a good case, and I suppose the parameters you identify work (good enough for government work, anyway ;) but I always have a nagging disquiet with the whole concept of a moral alignment.
I guess because, as a player, there is nothing to prevent one from suddenly changing alignments ("going postal" when otherwise conducting a sane, lawful character). I suppose that is all contingent on being a god to the characters.
Yeah, I suppose what I'm thinking is another framework, probably on top of the alignment mesh, to correlate psychological components. (I quite like playing chaotic neutral characters. No expectations. I'm also an ambivert —— both introverted and extroverted, depending on the situation.)

In answer to your question I can only say that I read a lot. Books, history, languages and cultures, people, philology, semeiotics, cosmological phenomena, … But I can neither confirm nor deny I am a psychologist. (And I am mildly insulted that you you would impugn me in such a gross manner —— I'm not that crazy! :)

I wouldn't read TOO much into D&D's alignment system. ;) At its heart, it's merely a convenient way of classifying characters into different spectrums of belief so that spells/powers/items that are only supposed to work on "Good/Evil/Lawful/Chaotic" characters can function without players having to devolve into arguments around the table about whether or not a certain action is Good or Evil. In general, D&D functions under the assumption that certain acts are ALWAYS Good, and certain acts are ALWAYS Evil, regardless of your motivations or justifications for doing them.

According to the Book of Exalted Deeds (a 3.5 supplement on roleplaying and expanded rules for Good characters), the taking of life of another sapient being is NEVER a Good act. It can be justified, in the case of Lawful executions and in self-defense or the defense of others, as you allude to in your example, but the act itself is never considered Good. The ultimate act of Goodness for that unrepentant mass murderer, in your example, would be to somehow convince the murderer to turn away from his path and embrace redemption. In theory, extremely powerful beings of Good could cause that murderer to spontaneously repent and seek atonement, but of course, such a change may be beyond the power of mortals to accomplish. In such circumstances, then the slaying or execution of said murderer may be the only option left to alleviate harm, although truly Good characters will always feel regret that a soul could not be saved from its own evil.

And therein lies the seduction of Evil even upon holy characters like priests and paladins. There may come a time when the paladin has slain so many evil foes and seen none of them ever seek repentance, and they DESERVE death for their sins. When that happens, the paladin/priest may come to ENJOY the act of killing, and in so doing they wind up beginning their fall from grace.

I actually approach the Law/Chaos component of morality differently from a lot of other D&D players. If Good/Evil determines what a character enjoys doing (i.e. helping/harming others), then Law/Chaos determines HOW they go about doing it. Lawful types are people who like knowing what they're going to do and how they're going to do it before they even get started. To give an example, if asked to assemble a piece of IKEA furniture, a Lawful type would read the entire instruction booklet before starting and follow it step-by-step every step of the way. In contrast, a Chaotic type would simply toss the booklet aside and go "I'll figure it out as I go." It's not that Chaotic characters are incapable of creating long-term plans, or lack long-term goals, it's just that they are the kind of people who are prone to abandon or modify those plans on a whim, and prefer operating on gut instinct and improvisation than sticking to a plan.

So, with that in mind, I haven't watched Dirty Harry myself, but it's entirely possible that depending on his actions, Harry might not be a Chaotic Good at all. His actions might ultimately not uphold "law and order", but that doesn't mean he couldn't have used a Lawful approach to do so.

An eloquent summary. I think your fallen paladin is a good approximation of Judge Dredd, who I would place at the Lawful-neutral border with Lawful-good: the law is the law.

It's one of those known unknowns, isn't it: can every soul be saved? Calvin (Protestant Predestination) would say not, but we can never know.
The character of Dirty Harry is certainly a lawful one. (By the way they were —— many —— novels before the five films were made.) I think you are correct in regarding him as Lawful-good, since he will often be compelled to act, and often against his better judgement, too, because he could not walk away from a malefactor.

He is probably on the cusp of Neutral-good, since he will certainly not follow a law he doesn't agree with, but always tries to do right. As you say, just because the character's moral compass doesn't strictly align with the legal system doesn't necessarily deem them immoral, just a law-breaker. :)