They toyed around with "visor vision" for helmets in previous games, but it never made the cut. It would get irritating having blurry, black borders around your viewing area while wearing a helmet.
Though personally, as a huge nerd who loves the idea of too much realism in my RPGs, I would relish the idea. For me, it would add a new layer of role playing to see through the slit in my visor, and take my helmet off for viewing comfort when I don't need it. Bonus points if there is a quick button that allows me to quickly equip and de-equip the helmet, complete with first person arms coming up and removing the helm, then storing it away. Double bonus points if wearing the helmet muffles sounds around you somewhat.
NPC - "Mmmmfff mfflll mmm mmmff..."
YOU - "What?" *Removes great-helm*
NPC - "I said, I saw a mudcrab the other day."