Here in Seattle it is the law to wear a helmet and they DO give out tickets for it. Also, working in medicine for twenty years and having cyclist friends I've seen some **** that makes me want to wear a helmet in the city even if it weren't the law. Maybe even if there weren't any cars around at all just because of the odds since I ride almost every day, most days at least 15 miles.
That said, I agree with you that in a lot of ways cycling itself isn't that dangerous. If it weren't for the cars and we were on dirt roads I don't think I'd bother with a helmet at all. Here in the PNW I always seem to be making wicked descents too. My next little bike camping trip is mostly on quiet country roads, part of it alongside rivers and sloughs so I'm thinking for once, I could do without the helm for part of my trip.