DD and Lunar,
The last time I fell off my bike was in 1996. No, that's not a typo. It's been 17 years and counting since I've fallen. I never had anything worse than road rash except once (pulled ligament on my left leg). Ironically, the pulled ligament happened in a crash at only 20 mph, and was my fault for fighting the fall, thinking I could land on my feet because I wasn't going that fast. The hard fact is the most likely thing to kill me while I'm riding is a car-bike collision (even that is fairly unlikely given that I ride defensively), and a helmet will be entirely useless in that scenario. I invite you both to do some reading:
http://www.cyclehelmets.org/ Helmets aren't all that effective. In fact, when you factor in the increased risk of rotational injury, some studies are showing they don't prevent additional injuries at all. Dave, you can't be sure the helmet saved you. I can crush any bike helmet an inch by hand. It may have prevented serious injury. Then again, since the helmet increases the size of your head, you may have missed hitting your head altogether without a helmet. Crashes are pretty complex things. If I had to choose safety equipment when riding, it would be knee and elbow pads.
I tried a friend's helmet once but couldn't stand it. The chin strap was irritating, it partially blocked my peripheral vision, I overheated, and it also affected the way I hear sounds. I rely heavily on sounds to hear what's coming up behind me. I'll just as soon never set foot on a bike again if I had to wear a helmet.
I just want to add that there have been some scenarios where technically a helmet saved a person's life, but they still sustained serious head injury, to the point they were pretty much vegetables. I'd rather just die than end up like that.
BTW, I pretty much always ride alone, barring the occasional cyclist I might come across who decides to challenge me. That probably decreases my chances of crashing about a hundredfold. Too much can happen when you're on someone's wheel, especially on the wonderful potholed streets here. Remember I grew up in an era when bike helmets didn't exist, even for children. I don't recall anyone I know getting head injuries on a bike, although I do recall a good number of skinned knees and elbows.
All that said, if I did trail riding, I most likely would wear a helmet because the irregular course would make crashes almost certain, and the speeds would be in the range where helmets are at least marginally effective. I also wouldn't need to use hearing or vision as much because motor vehicles wouldn't be on a bike trail. It's all about using appropriate equipment for the conditions. Nearly all my riding is alone on mostly empty streets after 9 PM, at speeds well above those at which helmets are effective. A lucky rabbit's foot would offer about as much protection as a bike helmet under those conditions.