It could be a stereotypse i'm not denying that but it could also be for headbanging, although people with short hair seem to be able to headbang perfectly fine :) I think it's more a metal/rock thing. If you don't have long shaggy hair you're not the part kinda thing LOL. But in my personal opinion i think it's because the ear plugs they use to protect their ears are crap, so they wear their hair long to have added protection :P LOL