The point is it saved injury to my head. We'll never know the extent of what damage to my head it saved, that will always be speculation. But hopefully you understand that was the only part of my body which experienced impact and which wasn't damaged. It was the only bit that had protection...
Well we can. Where are the major abrasions and impacts damage on the helmet shell that would have been on your head if the helmet had not been in the way? Its a bit like coming out of a car accident with an undamaged car and swearing the body shell had protected you from serious injury. If it saved your head from major damage, your helmet would have suffered major damage itself, if it doesn't have major damage then no major damage would have happened to your head.