I'd say that it isn't "all about" optimization because optimization is a means to an end, not an end in itself. You have to decide what you want to optimize for before you start after all.

If you want your character to be powerful, you can optimize a T1 Caster to make the game weep. If you want your character to be badass, I've seen Monk builds which can hold their own at T3-level play. If you want to fit a narrative role or have a cool RP idea, optimization will help ensure you're concept plays out the way you want it to.

So I'd say it's important, but not the goal or the metric of success.