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
I would say it makes headbangin' alot more fun...too lazy to cut it Perhaps. Money? No, guitars, drums, and other equipment is expensive so of course they can afford a haircut! Its the look, its fun, and its what makes heavy metal so unique and special!
It's a way to give off a heavy metal look. It was popular in the 80s, and it just kind of carried on to metal bands today or metal bands in genral, it's the style. Long hair just kind of resembles metal, like how cowboy boots resemble country music and how glitter and saggy pants resemble pop/rap. >_<