... They justify themselves.
I meant to comment on this post on Roger's Roles, Rules, and Rolls about the way experience awards changed over time. I've always disliked the change, but Roger's made it clear what's wrong with them in a way that I couldn't put into words: the "new experience awards" focus on rewarding the means of success, as opposed to rewarding success. You don't get a reward for solving a problem; you get a reward for behaving the way the GM wants you to behave.
The way I see it, if a magic-user picks up a dagger and starts slaying monsters instead of casting spells, that's not "bad role-playing". That's like adding your own challenge to the game. It's like when someone offers to play ping pong with one hand tied behind their back, or if they play Nethack under a self-imposed restriction like "Vegetarian" or "Pacifist". You shouldn't penalize someone for making their character more interesting.
That's actually the rationale behind the experience penalties for low prime ability scores. (Something else that disappeared with later editions...) I know a lot of people think it should be the other way around, that you should earn *more* experience for playing a Fighter with Strength 3. But, since Strength originally had no quantifiable effect on gameplay, the experience penalty is there to reflect the fact that you are trying to play against type: you are choosing to play a weakling warrior, or a dumb magician, for the challenge.