I know there are lots of stories out there about how helmets have saved lives and a lot of people also who defend not wearing a helmet. I’ll let the two photos of Alan Hartley below speak for themselves.
Alan fell in June at the World Cup MTBO in Hungary in April and cracked his helmet (which you can just see in the photo). He also damaged his knee and elbow and is still suffering from the knee. Although there was nothing visible at the time, since June the hair in the impact area has fallen out and is now growing back white!