Well, I can't remember exactly why I started to let my hair grow. Probably just decided to give it a go. I think that was about two or three years ago. My hair is now about 12-14 inches long, past my shoulders. It isn't straight, I'm not sure how to describe it.
Fortunately it has a couple of advantages, it looks okay when its a bit tangled, and my hair doesn't really need washing more than every week or two (I can get away with a month or more without it being yucky too me).
If I ever cut it off, I'll probably be tempted to go all the way, to a shiny top (at least to start with). I might also try to sell the hair, if I needed money (wigmakers might buy it).
I also tend to dress in black, with hooded jumpers. This came before my interest in metal, so it is probably coincidental.
In short, I've got long hair because that's what I've ended up with, and I have no particular reason to change now.