Personally I see the need for balance from a GM's perspective... if I can assume that a group of 4 5th level characters can defeat a single 5 CR encounter...its alot easier to prepare for the adventure.
When balance gets out of whack it takes longer to prepare a solid, and challenging, adventure for the group..and easier to under/over-estimate the proper CR to throw thier way. This turns games into a coin-toss of either walk-over encounters or TPKs.
On the other hand, not every option should be as viable as every other option. Sometimes the knife fighter just isn't the best person to send into the fight.
On the third hand, its the GM's position to allow the chosen chracter types to have an opportunity to shine..within reason.
{What I mean when I say 'within reason' is that if the game is declared to be a bunch of outlaw cowboys running from the law, don't try to bring a law-man into the group!}
So...there is a balance of mechanics, balance of role vs roll play, and the balance of GM style vs player style. I could spend entire sessions worried that something is out of place....
...or I could just play the game and have fun!
Anyway, what I have always liked about 3e is that it offers choices and options between which I am divided over which ones to pick...much better than earlier editions where my choices were basically race and class. YMMV