I think it's important to realize that there are two types of balance:
PC vs. NPC/monster balance, and
PC vs. PC balance.
For some, they're of equal importance. I personally don't think so. I think it's okay if, on occasion, the NPCs or monsters can clearly do something the PCs can't. I trust most of the DMs I game with, and I know that if something seems unfair, there's a plot-based reason for it, and we'll eventually have the chance to do something to even the odds.
If the NPCs/monsters are too unbalanced, however, it leads to a sense of frustration, like the PCs can't accomplish anything.
PC vs. PC balance ensures that no one character thoroughly and blatantly hogs the spotlight, gets all the glory, and receives the majority of playing time. This is, to me, far more important, since everyone deserves their fair share. However, if the group and DM decide ahead of time that one PC is somehow unbalanced, that can be okay, so long as everyone's okay with it and the DM plans encounters and stories to not focus on that particular character. It's exceedingly difficult, and few DMs can do it well, but it can be done.
Bottom line? Many people play games, at least in part, for the sense of accomplishment. Frustration, and feeling like you're second-best, is never a fun thing. Game balance is a major tool for avoiding that sort of thing. Is it the most important? Nope; the most important is a good DM and fellow players who can work things out so everyone's having fun. But balance is, perhaps, the most important tool for avoiding frustration that the game designers can include in the book.