Some people don't enjoy building optimised characters because it's too much work, and they're playing the game to have fun. Others prefer to optimise for concept or coolness rather than power.
Yeh, and some people even optimize their gimpiness.
What I keep saying is this: In a game like D&D, that has fixed rules on how you can do stuff, you have to build a cool character to be able to play him like a cool character. Anything else just won't work, and will just seem stupid. Substitute cool for bad-ass where necessary.
And those people that are completely into the flavour thing shouldn't bitch if their characters are weak. Except they still DO. So they are hypocrites, whereas I say that I don't want to play a character that sucks at what he does.
Now I must say you CAN overmechanise things. I have trouble roleplaying the aquisition of abilities for my Lvl8 Crasher/Blade/Barb/Crusader, at least in game. The strange thing is the best roleplaying experience I have in D&D is in my epic group, because I can basically do EVERYTHING, so I can just sit back and relax and play the character, while occasionally thinking about the mechanics. I mean honestly, there is so much cheese on that character that it's really not nice, but at least I can roleplay him nicely. For instance, he's a necromancer, and he rules a little country that is basically Pleasantville. Being able to do things like that... I can only do them because I can rightfully say that this character is poweful enough to do all that, and more. Flavour follows function in this case, because the function allows the flavour, and that should be a slap in the face of all those who still think you can't roleplay a character who is basically FULL of cheese.
Now this was sort of a rambling reply, I beg to excuse this.