like if your wearing a helmet that covers up so much of your face and only has a little slit for the eyes then you should only be able to see through that little slit. and that would give it a more realistic feel and give you more reason to take your helmet off when your not fighting.
It doesn't work that way, with a visor slit your field of view wouldn't suddenly be reduced to a tiny slit the same way as you don't see two circles in front of your face when wearing glasses.
A while ago i did some example pictures from a screenshot:
Eye holes: http://freenet-homepage.de/daniel-kay/wrong.jpg - http://freenet-homepage.de/daniel-kay/right.jpg
Visor slit: http://freenet-homepage.de/daniel-kay/wrong1.jpg - http://freenet-homepage.de/daniel-kay/right1.jpg
Barred visor: http://freenet-homepage.de/daniel-kay/wrong2.jpg - http://freenet-homepage.de/daniel-kay/right2.jpg
This is because your eyes focus is on the objects ahead of you and through the view openings and not ON them.
And yea I'm aware that the view cut outs wouldn't be as close to your eyes as glasses but you have to remember that this is a game and playability has to take a high priority, if you could only see 10% of the screen it would be an artificial obstacle. And I actually once made cut outs like this for paper masks and held them some distance to my face and i could still see through them quite well.
However all in all I'm not really in favor of helmet vision.