I definitely think multi-classing should be in place. When you look at the voters who state they dont use them/wouldnt use them, then how much does the ability to multiclass really affect you? Probably not much. As for balancing, etc, I like the way it worked in BG II. You could multi-class but it most certainly wasnt overpowered. Did it make for some interesting builds though and different ways to play the game? yes.
I do think its important that playing a single class be as powerful as a multi-class and that a comparison of multiclass vs single class be similar to comparing a barbarian to a fighter. Both have their strengths and weaknesses and its ultimately up to you, the player, to choose which road you travel.