i think it would help prevent some types of concussion, because often that occurs when the jaw is suddenly closed and then that energy is transferred to affect the brain via the teeth, or so i am told.
I wear one when i am playing rugby, you have to, and it does help in hard tackles or when you thumped in the face, but i think in skiing you have a real danger of getting a mouthguard caught in your windpipe, doing flips and shit -
Being upside down can't be good for keeping it in your mouth, i think the dangers outweigh the benefits on this one....