IIRC, helmets will increase the chance of survival by only around 15%. It will also reduce the risk of brain injury and especially trauma to the whole head/face region. However, it can also cause one to survive with massive head injury instead of just getting killed - what is worst must be determined by each on their own. Personally, I would have scraped off my face a couple of times without a helmet, and helmets have also saved my life and/or severe brain damage on several occasions.