I have just stopped wearing my helmet. When I came back to cycling some twenty or so years ago, I started mountain biking and wearing a helmet is a must for that in my opinion, when I started riding exclusively on the road I just carried on putting the lid on. It just felt odd riding without a helmet.
The reason I've stopped wearing one is that I believe helmet use has normalised the idea that cycling is dangerous. I'm with Chris Boardman with this and while I know on my own I won't change anyone's opinion, the journey starts with the first step and all that.
Around here the biggest danger is close passing motorists on the narrow lanes and herds of cows coming the other way! Having a helmet won't save me from either of those
Purely anecdotal and not scientific in any way but I've only had one, fast, close pass (on the main road) in the time that I've been lid-less. Further supporting the thought that wearing a lid encourages close passes.
It also means I get to wear some nice cycling caps
It's funny what you notice but almost all cyclists wear helmets. The only exceptions seem to be one or two of the older roadies who I assume have just never worn one and the guys on their way to work wearing normal clothes on bso's. It has become entrenched cycling=helmets.
I've nothing important to add to the discussion but I wanted to share my experience.