There’s a lot of confusion here.
Yes: helmets can reduce the severity of some crashes. They are not a magic cure-all. They are designed to withstand low-speed crashes (the standard test drops them from a height of 2m onto the ground).
http://www.bhsi.org/testing.htm
So if that’s what your crash involves, it could help.
But that’s *not* what this is about, which is whether they should be compulsory. The answer to that appears to be *no* because the large number of people who don’t take up cycling, and so suffer ill health as a result, more than outweighs the small number of people who are injured by low-speed bike crashes involving their head.
That’s probably too complicated for most people, I expect most of you gave up reading at the first sentence, so everyone should carry on talking utter tosh for the next few pages.