He's not really leading the army, that's you. But anyway. He can turn his head or have people watching his flanks to inform him of enemy movement. There's also plenty of openings for air. Fight properly? Well, you pretty much trade visibility for protecting your noggin.
Heavy armour like that is not without problems, but living through the battle is considered more important.
Consider your point of view in both games. How much time do you spend staring at your heroes in KAII? (Or similar games for that matter. Assuming they have any.)
I would still go with the helmet in Skyrim, but you are much closer, I can at least understand it a bit better there.