That being said, NO it should not be mandatory. But I would advise it. I was given an expensive skiers helmet for Christmas a few years back. Not real crazy about wearing it at first, but it did without a doubt prevent me from being injured when being run into at full speed by a snowboarder. The collision knocked her out cold, She did not have a helmet. I was blown out of my skis about 20 feet but was unhurt except for a few bumps and bruises.
Actually it was determined. She had a blood clot on the back of the skull, without the pressure being released it presses on the brain until it squeezes off the critical blood flow and the brain dies... releasing the clot would have released the pressure.