I like the idea. It'd make players think about balancing visibility and protection...
Except, with helmet design,
that's not an issue. Human eyes adjust around such obstructions. I like the effect of the New Vegas helm thing, except I feel that was a bit too restrictive... increase the size of the viewing field so just the very edges of the helm show up would be about right.
There is one, and only one, benefit to not wearing a Helmet, and how big it is depends on your personality:
You get to show off your heroic visage and awesome haircut. That is the
only benefit of going without a helmet. Any further restrictions would be unrealistic.
Even a Closed helm with tiny eyeslits wouldn't obstruct vision seriously. The most realistic way to implement it would be to just make the edges of the view slightly elliptical, and, if they could pull it off without it looking too wierd, an almost-impossible-to-distinguish hyperbolic divider down the middle of the screen (Like, 10% opacity at most), and it slides away from whichever direction you look before resetting to standard, as well as having the eliptical edges narrow in the direction you rapidly look (Slightly, not enough to make looking in that direction counterintuitive), and widen in the opposite direction. If Depth-of-Field is added, some cases of double-vision or images imposed over the black should appear.
But, for the most part, just a little bit of helmet at the very edges could be implemented, out of the way of normal sight. All it would do is aesthetically reshape the view screen, not restrict the view, and let you see that "Hey, I'm wearing a cool helmet!" even in 1st-person.