To the non helmet wearers, I'd be interest to know what age you are, and any of you started cycling in recent years, is the reason for not wearing one on the grounds of head injuries or just that you really do not want to wear one for other reasons
I'll answer, although I'm in the middle group in that I wear a helmet sometimes.
So, I commute in a helmet, I do it because it gives me somewhere to put my camera and rear light that is more solid than a hat. Also I believe it accentuates my head movements, so a head shake at a car about to pull out from a side road is likely to be easier to see. Finally commuting is where I ride in the densest amount of traffic and on the crappiest surfaces, generally at a reasonable (for me anyway) speed because I'm specifically going somewhere. So I figure my chances of an accident are higher. If I come off I don't want to scrape my face down the road, or generally get graze injuries on my head (having seen them from my Moto X days when people wore open face & face masks) so I have the helmet to possibly stop those, I'm happy to take the risk against rotational injuries. I don't expect the helmet to ever 'save my life'. Just the same as I don't expect my gloves to, nor my glasses. As I'm wearing a helmet, I wear a MET Parachute (mk1) which is a weird enough choice to prove I'm not copying any roadies.
I leisure ride in a baseball cap. This is because I don't bother with lights and cameras, I tend to be going slower, I'm often with friends or family and we are chatting and pootling along, Also I've chosen the where and the when to enjoy the ride so it's unlikely to be on leaf strewn, slippery, manhole infested roads dodging around cars, scooters and lorries. I wore a helmet when 'training' for the Ride London, because I'd be doing it in a helmet (their rules). I didn't wear a helmet on the Freecycle the day before. I do a tiny bit of mountain biking, I wear a helmet for that too. My basic kit requirement is gloves, glasses, hat, helmet (specifically in that order, I can't remember the last time I rode without gloves or glasses and I do feel 'wrong' riding a bicycle without them.)
I'm perfectly comfortable either way. Like others, I have a motorbike background and for that wearing a crash helmet is second nature.
I rode as my only transport as a teen in the 80's and didn't own a helmet. I kind of paused when I got a car, then did a bit more, including commuting in my early 20's. I bought a helmet but it was a horrible uncomfortable polystyrene abomination and I only ever wore it under severe duress. I then stopped riding completely, largely due to getting out of the habit after a nasty winter I drove through, and didn't start again until my early 40's, I'm now 44. I bought a helmet when I got back in to riding because I believed that is what you did nowadays. I chose not to wear it for everything because I felt sometimes it made sense and sometimes it didn't.