I know what I like and I brew to that style. I'd probably get slammed in a competition.
This "I don't brew to style" objection doesn't hold much water. As I judge, I have marked beers down for not being to style (although beers that score low are far more likely to have process flaws than style flaws). In most cases there is a better style to have entered it in. The problem isn't brewing to style it is knowing how to enter the beer you have and not the one you thought you made.
I've got just about enough points to be National, nearly all of them judging, and I can't think of one time I had an outstanding beer that wouldn't have scored high in any of the categories. It just doesn't happen.
My point is, that I don't care to get anal about a particular style. I taste a beer that I like and then make adjustments to my particular tastes and to what I have in my inventory. If making a beer just to satisfy a judge or to an exact style is important, then go for it. It's just not my thing.
Thats fine and I don't know any homebrewers that brew to satisfy judges (though I know some that enter and win a lot, which does not imply that competitions rule how they brew, they just make good beer and win a lot). I certainly wouldn't recommend it. I'm just saying that if you brew a technically great beer that "works", there is a category in which it will medal so people shouldn't feel like competitions have no value if they aren't some sort of mythical style nazi.
Competitions are always optional though. I don't try to talk people in or out of doing them.
I agree that the feedback can be of variable quality. My recommendations to people with this concern are typically to enter the best judged competitions (which are the hardest to win, in general) and learn how to parse the feedback. If you like a lot of crystal malt in an IPA and a judge says that it is too much crystal you can recognize this as a difference in opinion or interpretation and ignore it as you know what you like. If the judge says it has diacetyl you should think about whether or not they are right (did other judges pick it up, do you, do you know how your diacetyl threshold compares to others).
I think QA is important in any brewery and competitions are one of the many ways for a homebrewer to engage others in the QA process for their beers.