I don't think I could be allowed to GM a game where a space wolf or another marine runs around with their helmet off. I would be making called shots for the head all day long on them. Sure I would miss more, but a hit would be an automatic +8 damage. Or just use a flamer type weapon to autohit them in the head, even better.
Imagine a space wolf hit by a flamer with his head on fire running around screaming due to failing his willpower roll, as his whole action for that round, and taking another 1d10 + 4 damage with just his toughness for defense.
I know it is in the lore/pictures for marines to run around like that. It just kind of flies in the face of common sense for me.