It depends upon the game I would say. Personally I like that there is a variety of systems.

Realism definitely has its place, because the more realism, the more immersion generally. But dispensing realism for abstraction or other reasons allows many fun mechanics you wouldn't otherwise have.

Most strategy games that uses RPG mechanics uses MP, like most RPG's does. But the first Heroes of Might and Magic had a different system. A hero only learns a spell temporarily and can only use it as many times as his knowledge stat allows him. If a hero has 5 in knowledge he can only cast a spell he has learned 5 times and then it is gone.

In the other games in the series they changed the system to a MP based one where a hero instantly learns a spell for the rest of the game when he comes into contact with it. In these games you refill your MP by drinking from a well or spending the night in a town with a mage guild. This system is more fun and better in several ways, but there are some nice things about the system in the first game also.

Since you can only get back certain spells from certain towns, and those towns can be far away, you are forced to use
a wider variety of spells than in the later games. Spells are also somewhat less powerful because of this.

It would be interesting to play HoMM3 with this system since that game has huge number of spells, but some that are so powerful that they are the best choice in most circumstances.

And this is subverted by a couple big enemies having "on death" spells so the best strategy is to osmose them to 0 MP before killing them so you survive their on-death Ultima.

Except that the one I'm thinking of has too much MP for that to be feasible, so it's easiest to just cast Reraise on a party member to allow them to survive it. (In FF6, Reraise can prevent a party wipe; this is unlike, say, Rudra no Hihou, where Reraise will not.)

Going back to that boss fight, for a moment:
* Life 3 (Reraise) isn't hard to learn if you do the Phoenix Cave first; it just takes time, but only one character needs to learn it. (Also, if your party contains Gogo and someone who knows Reraise, Gogo will be able to cast that spell (but for that particular dungeon, give him the boring commands Fight, Magic, and Item.)
* Another approach is to jump up in the air and then kill the boss with that jump attack before everybody has landed. (Of course, this involves finding a way to jump, but such a method might be found if you re-visit that island where Cid is.)

By the way, the "die at 0 MP" property is inherited through Rage.

As for attacking enemy resources, I find it much more practical in Final Fantasy 5, where there's a spell that takes away half the target's MP, not to mention a late-game way to make a character's physical attack steal a quarter of the enemy's current MP per hit. (It's funny to see a certain boss try to self-destruct.)

Also, speaking of enemies running out of MP, maybe I should mention the demonite from Dragon Quest 3, which is an enemy clearly meant to scare those who've played DQ2.

That's not really a tweak, that's just the other side of the same bad coin I don't like (having to conserve spells throughout the dungeon -> having to conserve physical attacks). Warrior classes will be as crappy as some mage classes currently in some RPGs.

If all abilities have to consume 'points', I'd rather they have a unified pool for that. I don't know any RPGs like that, though; The closest thing is FF 13 where most of your abilities (physical and spells) only consume 1/3 (low), 2/3 (mid), or 3/3 (hi) of your ATB bar.

You can find a somewhat similar system in Pathfinder: Kingmaker. It's based on Pathfinder system, which is based on D&D, but some spells to be cast require certain resources (apart from standard usage per day). It's not a pure "to cast you need a resource" system, but it's the first thing that comes to my mind. I found this quite redundant. Usage per day is enough of a limit for caster characters in Pathfinder: Kingmaker. Maybe this system would be better if it focused solely on resources without the limit of usage per day. But of course that wouldn't be Pathfinder anymore. :)

That's an interesting possibility. But it also has disadvantages. It eliminates the need to rationally manage the MP. I think that a game needs this aspect in some form. It could work however if there are some other limits employed (such as ability cooldown for example).

That's a nice mechanic. I'd like to see it more frequently.

Reminds me of the need for reagents to cast spells (in addition to MP) in Ultima games starting with Ultima 4. I *hate* this mechanic, and wish it had not been implemented, or at least only implemented for the most powerful of spells (like those that require Mandrake Root).

It is because of this that I often choose to replay Ultima 3 instead of Ultima 4, even if Ultima 4 is superior on many respects. (Then again, Ultima 3 has the best and most balanced class system in the series.)

The Alliance Alive does this; everything uses SP except for consumable items.

Also, Romancing SaGa: Minstrel Song is like that for BP; it just has a few attacks consume separate, longer-term, resources in addition to the short-term resource of BP.

Some games, like Dragon Quest 8, use MP but give some physical skills MP costs. (DQ6/DQ7 gave all physical skills an MP cost of 0 (though I hear it may have been changed for the 3DS and mobile versions of DQ7).