That's an unreasonable response to, "I don't see a great demand."So you've never ...
Oozes, animals, vermin and zombies are of non-human intelligence; qualitative difference being the significant factor. Blob and Sabertooth have different personalities than Magneto, or than each other for that matter.
Nor does it follow that NPCs and "monsters" are bound by the same rules as PCs. Indeed, that's one way in which 3e stands out as odd in the D&D context.
One can choose to treat D&D as a comprehensive and precise mathematical model, its accuracy taken a priori. Heck, one can make alignment dictate behavior as well.
I just don't see a big payoff in such a rigid view. When it's fun entertainment, play up the shticks; when they get in the way of fun game challenges, set them aside. "My character is stupid and foolish and obnoxious" ranks with "My character is Chaotic Neutral" as a "role-playing" excuse for being a rotten player by certain standards.
So, I'll go for the rule of "Use common sense and don't be a jerk." That's pretty loose-y goose-y and not Terribly Serious, which may be part of why I like it.