Hair styles is a different thing, they are done with long hair btw. As for long hair on warriors - Scottish highlanders come to mind, vikings did not have haircuts either as well as many other people and cultures that was very warrior orientated (Russian people did not cut their hair short, neither did majority of the medieval Europe). And no you don't have to have a perfect vision to block meter + metal rod coming your way
It's veeery hard to miss form the close up distance it is used at.
While samurai kept shorter hair... I'd argue they were some of the fastest and most refined warriors out of necessity due to their weapons of choice. The point is, longer hair is not good for hand-to-hand combat (in ranged there is only one disadvantage). The Dark Ages present a unique situation where weapons became heavier and more blunt. Remember that most medieval warriors were wearing plate and chain armors as opposed to lighter ancient leathers and furs. Vikings were largely bandits and pirates known for shock and awe raids and not so much "warfare," though that did take place. In most cases, villages would flee at the sight of oncoming Viking ships--exactly how the Vikings wanted it since they could pillage without much resistance.
And I'm willing to bet you haven't done much hand-to-hand combat yourself as you'd know it
isn't hard to miss. When people are swinging at you, be it with a fist or sword, you're thinking about a lot of things--balance, fatigue, how to overcome them before they overcome you, not getting hit yourself. I'm will to bet that the millions who have died in combat since the Egyptians until the present would disagree with you.
I guess what I'm saying is that from a tactical standpoint, longer hair doesn't make sense. From a gamer standpoint, I say give me as many options as you can and I'll overlook things like tactics.