Anecdata: When I was about 18, three teenagers were driving to a football match. I knew two of them. On the way they crashed head on into another car at 70mph. The driver and front seat passenger, belted, escaped with minor injuries. The rear seat passenger, unbelted - it was only compulsory in the front back then, in fact there were still many older cars without belts in the back - was flung about inside the car and suffered serious brain injuries. He still can't walk properly, in fact he'll never have a 'normal' life at all.
Would a seatbelt have saved him? Seems like it. A helmet? No idea. Does this prove anything? Doubt it.