I don't wear a helmet because I 1: ride a recumbent, and the lines of force needed for me to end up hitting my head are unlikely and 2: my strategy for dealing with temperature changes involves adjustments to my head coverage - adding/removing hats and the like; often I don, remove, or change hats several times on a ride in order to adjust to road and weather changes.

If I road a road bike, or a MTB on trails? I would wear a helmet. Much more likely to hit my head with one of those. I don't require others to, though.

That said, I do not ride without an ANSI II reflective vest on. I twitch at the sort of recklessness embodied in people who ride their bikes without donning reflective clothing.