I played once last winter with my nephew. Man, it get's steamy under that mask/shield. The place where we played said that if anyone took off their mask for ANY reason during play, you'd be kicked out. Sound advise, so it would seem.
I was watching my cousins in a tournament last spring when one of the players tripped and accidentally squeezed the trigger. The pellet hit me in the side of the head and knocked me for a loop. I can see why they require the helmets and face shields.