I see what you are saying.
Nothing is foolproof. The only way to be 100% safe is to be locked away in a padded room. That is no life at all, and I don't mind taking risks. For years my dear old Mom begged me to stop riding, citing the danger. I told her I would rather die doing something I love than sitting in a chair in front of the TV.
My biggest complaint with bike helmets is that they don't offer protection to the truly delicate parts of the human head, the face and the base of the skull/top of the spinal column. I doubt I would wear a full face helmet though, no matter what, not in the hot months anyway.
Most of my bike accidents have not involved head impact. I usually catch myself and manage to roll, but not always. To me the helmet is most useful in case of impact with a car in which case the cyclist will rarely have a chance to manage the accident and the velocity at impact is liable to be much higher than most bicycle only accidents.
Perhaps I will go back and read this whole thread when I have time.