I ride with a helmet 90% of the time. If I'm clipped in and/or riding with vehicle traffic or on uneven terrain, I wear it, which is most of the time.
After cleaning/tuning my bike and riding it up and down the complex to check it out, I don't wear one (I'm usually in flipflops or sandals of some sort), or if I'm running across the street to the market to quickly grab a few things I typically don't. If I'm on my cruiser tooling around on the bike path, I may not wear it, either.
I'm like Pax, and it's a risk tolerance decision. To each their own. I rode horses for years and fell off, was bucked off, rolled over top of, trampled, etc, and I never wore a helmet during all of that (I used to race barrels), and I doubt even now I would ever wear a helmet while riding a horse. I do, however, wear one whenever I snowboard - I suck at it, and fall every time I go, and have rung my bell a number of times out on the mountain where I'm certain a helmet has saved me, at the very least, from a bad headache.
MY risk is a lot higher on the slopes than on the trail on my cruiser. I've been hit more times in the head/face while playing softball than on my bike and I still play shortstop (without a helmet). There's inherent risk in everything we do, and it's up to individuals to decide what's right for them. I truly believe in helmet education and advocacy, but I am against a law that requires people to wear them. I also don't believe in seatbelt laws even though I buckle up every time I get behind the wheel (no maybes or sometimes about that). If someone chooses not to, that's their decision. My risk factor decision doesn't give me the right to impose that belief on other people, and that's what I think seatbelt and helmet laws do (insurance rate factors aside). I see it like this - if seatbelts and helmets save lives, and there are laws that say people have to wear them, then they ought to outlaw things that are bad for you like cigarettes and alcohol which kill a lot more people than a lack of the other two (while I don't smoke, I do drink, so I'm not advocating for that! But I see it as the same thing )
Note, I am not a closet-non helmet wearer. Most times I wouldn't ride without one. I don't wear it because I feel pressured to do so, like our European transplant explained. My activity and the risk I assess to the situation helps me make that determination.