Proof?
<img src="http://www.flamesonmytank.co.za/photos/busthelmet.jpg">
I've always worn a full face. When I was getting ready to purchase my first helmet a friend asked what would happen if I planted my face into a curb... so that pretty much sealed the deal.
The above pic is off of a helmet website, it wasn't me wearing that lid. But I think it shows what will get damaged in a good wipe out.
Ride safe.