NPR had a little spot on if helmets are really effective or not in preventing head injuries. Not really trying to have another helmet thread debate. Just throwing it out cause its an interesting listen.
Heres the way I see it. If you land 50 foot park jump on your head will a helmet help you? Probably not. If you are on a rail and slip back will your helmet stop your brains from falling out? Yes. Will it help if you scrape your head on a tree or rock? Definatly. Overall: worth it