It's a matter of statistics, I think.
If you ride a bike one mile per day, in conditions where you aren't mingling with faster traffic, and moving at slower speeds, then your risk is pretty low. If I rode on the bike paths in the Netherlands, I wouldn't wear a helmet either.
In my case, I ride my bike over 8000 miles per year. Many of these miles are on the same roads with cars traveling 60mph, in most weather conditions, and in day or night. I've whacked my head on the ground due to slipping on ice, but that's been it. The helmet definitely helped!
Some of my friends haven't been as lucky as me. One had a dog jump in front of his bike, knocking him onto the pavement, causing him to be unconscious for 10 seconds or more (and this was *with* a helmet). His helmet was broken up, but he lived (the only injury was a few cracked ribs).
Another friend did a face-plant because the carbon forks broke off. He struck the top of his head, and crunched up the helmet severely. Again, the only injury was broken ribs.
I also saw a gal mis-judge her speed in a turn, resulting in bouncing her head off the ground. The helmet was trashed, and she hit hard enough to have some short-term memory loss, but she was okay.
I think people should make their own decisions, but they should also be fully informed of what the risks and hazards really are. When possible, learn from the painful and expensive mistakes of others. Wearing a helmet is much less of an inconvenience for me than significant brain injury (or even having a broken nose or losing a few teeth).
better safe than sorry...
Steve K.