Well, I also realized that if you don't wear a helmet and you hit yourself on the side, your head might cock somewhere extremely quickly, and could result in a neck injury. With a full face, not only will the helmet reduce the impact (saw a demonstration, this guy used a hammer, on a guy with a cusion on his stomach then a board on top of it and because of the hard-soft combo, the force wasn't too much. Same with helmets: fibreglass with foam) but it might also stop the head earlier before hitting the ground, thus suffering a less accelerated whiplash.