Context is important here. We were not speaking of the fellow knocked off in a "right hook" or other low-speed incident. We were speaking of the high-speed "hit from behind" scenario, which appears to be becoming more common these days, at least in my neighborhood. "The norm" is nothing more than my personal experience, which is not vast, but is not inconsequential either.

Just for clarification: in my experience, when a cyclist is struck by a car at high speed, there are multiple causes of death. A helmet, in that situation, is superfluous. It is certainly possible, though outside of my personal experience, that a cyclist could be struck by a vehicle at high speed and not suffer life-threatening injury to any part of the body other than the head, and that the life-threatening injury to the head could be mitigated by a bicycle helmet. It is my opinion, based upon my personal experience, that such an incident is so rare that trying to prepare for it is similar to trying to prepare for a meteorite strike. IOW, you are welcome to prepare for it. I don't see the need.