Based on that argument, we should be wearing a helmet all the time, in case we trip and fall while walking. :-D

To me is a personal choice, and sometimes a matter of comfort. Group rides always require them it seems, so need to be comfortable wearing one in order to enjoy those rides, thus wearing one on most other rides helps with that. Helmet also provides some level of protection from the weather - wind, rain, sun. So I believe there is a side benefit there. I wear one on any ride where I'll be with auto traffic, when leading any rides with minors. I don't always wear one on the lightly traveled MUP by my house when on a cruiser/comfort bike, nor in my quiet neighborhood on a cruiser/comfort bike.

To each his or her own on this - just like whether or not to wear bike shorts, clipless shoes, or a cycling jersey. The helmet should not get in the way of someone enjoying a ride