The first time I ever wore a helmet on a bicycle, an old woman intentionally ran me off the road. I whacked my head on a telephone pole. My head was ok; the helmet was not. I do not even ride in our driveway without one now.

As to the comparatively flimsy construction of bike helmets versus motorcycle and automobile racing helmets, the potential speeds and intertial mass involved are much less. I've also noticed that some of the downhillers in the area have started wearing bmx type helmets which do give better coverage at the expense of being considerably warmer.