I skimmed an article a while back, in which a neurosurgeon claimed that helmets do little to protect the brain in cycling accidents. I'll see if I can find it, but I seriously doubt that is true. But...
When I was thrown into the street, my broad backside took the brunt of the impact and my head hitting the pavement was kind of a secondary bounce that cracked my helmet in three places. It's possible that if I'd gone down head first (and not in the recumbent position) there would have been significantly more damage.