It's interesting the number of people in this thread that share the same sentiment (wear helmet off but not on road).
I read a while back about a pro skateboarder practising grinds on stone wall when it caught and stuck throwing him forward, the speed of a grind cant be too fast i dont think but the collisions his head made with the lamppost killed him... he wasnt wearing a helmet. This being said i don't wear a helmet, silly i know but vein as it is i dont want to look like a complete tool though i should have one.
though true being hit by a car the helmet wont do anything for initial impact id assume if its hard enough to throw you from the bike you arnt going to be able to control your landing, even be concious for it so i think it would still be beneficial.
In that situation it would be covered by the "off" road scenario by most in this thread, for that very reason. Take me, very rarely fall off my bike on the road, in fact there is only twice, once cycling through a town centre and hitting a patch of ice as I was going round a corner, slam!

Second was heading down a hill along a bendy country lane at 20+mph, spending too long staring at a road just before the corner and forgetting to turn until it was too late...Crash into a bush..
The first would have been a no helmet situation for most, however that is the only time I've ever fallen off in a town, the second I should have been riding with a helmet as most of that trip was off road...

Off road however I fall off all the time and a helmet really would help. Thing is I also think a helmet means you take more risks, which isn't necessarily a good thing. I found the same skiing. I got a (3/4 face) helmet because I thought I was going too fast without one. Now I have a helmet I go even faster...
So yeah, on road, no, because it's unlikely to make a difference with all the murderers on there however off road definately.