My optimization 2c.

Personally, I optimize above average, but sometimes, you need to self-regulate your characters so you don't outshine the party and someone doesn't have fun.

Let's say you're a God Wizard build. Just because your character can win every encounter by himself, doesn't mean that he may have some crippling social/psychological flaw. Perhaps he's ADD and you choose to only act every other turn. That isn't a mechanical flaw, but you choose to act that way in order to let your party's Human Fighter have a little fun.

Or maybe your God Wizard is a pacifist. He never wants the killing blow, so he always hits another target, again, letting Mr. Fighter have his fun. Just because the Tier System exists doesn't mean the player can't act unoptimized in order to not make the party unhappy.

And I personally have played the "Scooby" (for those who don't know, the Scooby is a comic relief character who is COMPLETELY sub-optimal, but is the lovable character who no matter what mess he's in, the party still wants him to be safe. The term comes from Scooby Doo (obviously). It's four semi-optimized party members and a half-retarded talking dog) and I enjoy being so sub-optimal that the party is left confused.