What is not addressed is the problem of coup-contrecoup brain injuries. Helmets do little to alleviate head injuries of this type.
Please read this post and the following thread for a good explanation of why helmets help sometimes and don’t help others: A simple bump on the head can kill you. Read the comments for the full story.
I think that there is a reluctance to admit that helmets do not prevent all, or even all types, of head injuries because it’s thought that people won’t wear them if they don’t work 100% of the time.
The other side of that argument is that people who wear helmets may think they are protected to an extent they can take risks they ordinarily wouldn’t.
The take-home message here is to always wear a helmet and to always be careful because a helmet can only protect you against some head injuries, not all.