There's a lot of disadvantage in having long hair. For example, fleas and other vermin could infest it. Another is that you'd look like a faggot fairy in it, that is the case at least for some people.
Long hair would probably look good on you if you had a body builder physique but if you're a skinny motherfucker with a potbelly, forget it.
And lastly, if you're slugging it out with someone, your long hair is probably your achilles heel as it could easily be tugged and pulled by your opponent so he could beat the fucking shit out of you