The only reason they don't wear them is because it is not required. Seat belt hindered drivers in the 70"s but somehow drivwers got used to them.. I remember when OSHA wanted to put guards on table saws and other owood worjking equipment. Everyone complained! Production managers claimed "The workers can't get their work done", "It is more dangerous!"
They (mouthguards might not work every time, seat belts don't either, but if it was your kid and it saved him or her even a busted lip or tooth, I;d say it was worth it.