We all take our lives into our hands on public roads; whether as pedestrian, cyclist, or vehicle driver.
If a 40 tonner runs over me, a helmet isn't going to stop the Grim Reaper paying me visit..
On the other hand, if the 40 tonner close passes me causing me to fall off, then based upon the bits of me which have been hurt in previous bike accidents or incidents, I would expect to hurt, in descending order, the following,
Hips
Knees
Elbows
Wrists
Shins
Forearms
Testicles
Head
As I said in an earlier post, I have no intention of looking like a 7 year old dressed by its parents for its first trip to a skateboard park, simply to protect everything which could be hurt.
Yes there is always the chance that I could hurt my head whilst cycling, but I have genuinely bashed my head many more times and far more severley whilst not riding my bike. - I would include in this, the head of a 14lb sledgehammer whilst it was being used to drive in a post I was holding.