Well its not something that I cant do without, it really depends on the man. Hygiene comes first, and not all hair types look good long.
That said, , I love long hair on men.

My favorite is shoulder length, I just think it looks manly. I am not a big fan of very, very long hair on men, I think it looks girly and not many men can pull it off.

I often tease Hubby that I married Him for His hair, lol. When I met Him it was rather long, but it looked good; sometime later He cut it short (I will do a blog post about that, soon. But I didn't think it was funny at the time, lol); these days He keeps it at around shoulder length, and as Canadians say that looks awesome, .

Why I prefer it? Hard to say, it could be just aesthetics of it. Like I said, hygiene is important to me and that makes it clear pretty fast. Also, for guys like my hubby whose job is to play a big, bad businessmen i.e. desk worm, lol, it gives a certain primeval masculinity, and a sense of danger that comes with breaking the mold.