As [MENTION=49017]Bluenose[/MENTION] said, there is extensive reference to balance by Gygax in the AD&D books. It is also called out as an important consideration in the Moldvay Basic rulebook.They don't. As long as a class is fun to play and isn't rendered useless by another class, I don't see what the obsessive fuss is about that makes "balance" a central design goal to the point that it hurts other aspects of the game.
There's a certain amount of balance fetishism that suddenly appeared circa 2008 that I didn't see before and that I really don't understand.
My own view is that balance - conceived of as comparable mechanical effectiveness across the classes in contributing to typical scenarios and situations - was seen to be less important in a certain sort of 2nd ed play which downplayed the action resolution mechanics altogether for a mixture instead of freeforming and GM fiat.
Another feature of 2nd ed, and one sees it coming up also in this thread, is that the GM is said to have a special job to do in maintaining balance, by telling players how they may and may not build their PCs. In my view that sort of GM approach is not really viable in a game which expectes PC building to be a signficant element in the game - whether because part of the skill of playing the game is building an effective PC (this is a part of 4e, and I gather quite a big part of 3E for many people), or because part of the point of playing the game is for the player to control the PC and the PC's development (this player protagonism is a big part of 4e, and presumably a part of 3E too).