It's just one of those opinions the snobby hunter riders try to force in. There's no rule against roached manes at all.
And I haven't hit any big shows with my young gelding yet, but we've hit plenty of schooling shows at different venues (ie. different shows), and have always placed, even in the much bigger classes (25+). And my horse is the kind that gives those hunters heart failures. He's not your plain-jane colour (he's a dun), he stands out and my "attire" consists of butter-coloured breeches, mustard show shirt and black show jacket (apparently that's a fashion faux pas!). And slapping a bigger insult to hunters apparently, I don't braid my horse's tail and he has a very noticable mohawk. Never, ever has a judge said anything about his mane especially. Every critique I've gotten has been things to improve our riding (ie. my position, his momentum, etc.). But I show for fun, and I guess we're just too stubborn to try and conform with the nazi-hunter opinions. It sure hasn't affected us yet, much to the disappointment of the peanut gallery!

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What doesn't kill you; is sure to leave a horrible scar. 
Success is not final, failure is not fatal: it is the courage to continue that counts.