The thing you are missing is that there are FAR better ways to encourage or even enforce 'difference' and uniqueness in classes than arbitrary, often nonsensical prohibitions and restrictions. Want Paladins to be distinguishable from Fighters and such? What you do is to grant greater/faster aptitude with certain weapons and equipment (for example, sticking with D&D's already existent proficiency system, allow Paladins to achieve 'Grandmaster' skill with two-handed swords but restrictions on how good they can get with bows, etc. Then of course you have the special abilities like 'Lay on Hands' and such. D&D 3.X does as well as can be expected in some of this stuff considering the ruleset they are stuck with. False. I realize you can 'house rule' ways to patch things up but AD&D clearly states that mages cannot use any weapons but staves, daggers and darts and Clerics cannot use any but blunt weapons, regardless of deity/religion. Again, this is untrue. In the 'real rules' there are simple arbitrary prohibitions and restrictions for races, classes, gear and levels (for non-humans). LOL...okay, lots of problems with the above. 1) You do realize you just said "there is nothing stopping you..." followed by a descriptive explanation of what would stop you from using prohibited weapons, right? 2) In BG a Mage cannot use a sword or flail. period. Nothing to do with the threat of electro-shock. Unless you are a multi-classed fighter/mage or some such, you simply cannot use, touch, equip or wield certain weapons. 3) Do you really not see how much of a desperate rationalization you are offering here?! You are basically saying 'No waitaminute. It now makes complete sense that Elric cannot exist in AD&D because no sorcerer can even pick up a sword because I can imagine the sword might electrocute Elric for trying to use it." ?!

This same applies for other absurdities such as thieves being unable to don plate armor as well. It is fine to penalize thief abilities while wearing heavy armor/being encumbered but the way AD&D does this makes no sense and is simply lazy design. Not in AD&D or Baldur's Gate. You can imagine whatever specious reasoning you want for this but the real truth is that AD&D is a primitive RPG design full of crappy, lazy 'solutions' to perceived problems. This was a latter rule revision TSR offered once they got tired of trying to deny they had done things wrongly the first few times and in the Baldur's Gate games they implemented (in BG 2) "Kits" which allow for different types of Cleric (a very few different types at least). In BG 1 you are stuck without even THIS much.
They seemed to recognize that it would be impossible and dumb to try and come up with a new "Class" or "Sub-class" for every possible fantasy character build in heroic fantasy so they...added a bunch of new secondary character classes but called them "kits" . Oh...well okay then. I will just go ahead right now and roll up a human fighter-mage-thief-cleric so I can create the character I want to play and just ignore the fact that, even though my character concept had nothing to do with him being a priest or a thief, he is now stuck with those 'classes'.
Oh waitaminute...no I WON'T do that because for some bizarre reason humans cannot multi-class! So maybe I will 're-imagine' him to be a dwarf fighter-mage-thief-cleric. Oh except I cannot have four classes in my multi-class build and even if I could, the mightiest dwarf fighter in ANY AD&D world is little more than dragon bait with his level 10 maximum (or whatever it is). Yeah...right. Non-humans in AD&D can be a whole single point better than a human in ONE attribute and maybe a bunch of points worse in other attributes and they can see in the dark better and maybe are a bit better at spotting secret doors in their native habitats. That more than makes up for the fact that, at their maximum levels they could not last 2 rounds with a 20th level human of the same class. Great way to encourage unique character building is to, in effect force non-humans to all be 2-3 classes, and pretty much the same exact builds for every race (i.e. elves are almost always fighter/mage or Fighter/Mage/Thief(or cleric) of some sort). I just cannot stand the idea of playing a dwarf who is an extraordinary warrior, sans magic spells or thief skills (/sarcasm). Except they DO matter even in BG1 where they try to get around the problems via artificial level/experience restrictions (as per the AD&D rules) since it usually makes more sense to be a multi-class elf or dwarf or whatever than it does to be a single classed human. Not in a well designed RPG. That humans would get steam rolled in AD&D and so they figured they must imposed arbitrary level and class restrictions is just more evidence of AD&D's poor design. +4 Straightjacket of Culture is not even usable by humans. ;)

But seriously, you don't think that the very notion that ALL of the human species(on ANY world) is lumped together under the "race" of "human a bit 'striaghtjacket-ing'? No physical, mental or proficiency differences between Vikings, Samurai, Zulus, Melniboean, Rohan (Rohirim?), Cimmerians etc.?