A bit late to this thread but its been quite fascinating.
My personal experience from some 45 years of riding is that a helmet would have made no difference to my injuries the couple of times i have had head injuries in cycling incidents. How do i know? well each time, one a convoluted ice fall and the other an over the bars ice induced crash, resulted in lower face scrapes, which of course only a full face helmet might prevent. Of the other falls, none resulted in any head / ground impact - even when i tried flying using a sunken man hole cover as a ramp, i travelled some 10m, put my shoulder out and got a few bizarre cuts and scrapes but nothing on my head.
I don't generally wear a helmet, only when event participation makes it compulsory or mountain biking where i consider there is a higher risk of hitting something (not that i've ever had more than a few abrasions from mtb offs). I'll also admit to having done some judo as a child
I have, like others have commented, banged my head many times where
riding a bicycle wasn't involved resulting in all sorts of lumps, bumps and contusions - a lot have been me walking into things, cupboard doors, signs, bus mirrors (a regular occurence at one time
), low doorways / ceilings, bikes in workstands / displays and even a trike falling off a shelf (i used to work in a bike shop) as well as falls / trips / rugby etc, etc.
I'm sure some people will take more chances when wearing a helmet in much the same way that they do with 'better' brakes but what impact (sic) that has on injury / accident statistics will never be known - who would admit it even if they were aware?