I once had a wasp in a helmet vent, I felt something hit my head and poked a finger I to the vent. It then stung my forehead, which was not pleasant. However as a mountain biker for the last fifteen or so years, I've lost count of the number of times a helmet has saved me from serious head injury. I also wear a motorcycle helmet, and would even if it was not a legal requirement.
I'm not sure I believe this study about wearing a helmet causing drivers to come closer. It's always held up as a definitive piece of research by the anti helmet legislation brigade.
That said, I don't think bicycle helmets should be a legal requirement, too many civil liberties are legislated against by the so called Nanny State already.
Also, modern mountain bike helmets have increased protection around the base of the skull, but road helmets do not, the reason road riders resist this is because in their aerodynamic "tuck" their back is almost horizontal with their head raised, which would create discomfort if the helmet interfered. Mountains bikes are typically more upright and riding positions are more dynamic.
Also, the peak on the front of any quality helmet is designed to detach itself in the event of an impact and as such is not strong enough to knock the helmet backwards off your head, indeed misplacing a correctly fitted helmet is very difficult, although it does happen, even occasionally with well fitting motorcycle helmets.