Helmets discussions bring out the worst in people. You can find people saying things like "Maybe he (the cyclist) doesn’t want to waste good money on his useless brain!" when discussing the most innocuous video of cycling. Helmet advocates get quite upset even at helmets that are worn but not tight enough. They bring all sorts of arguments to the table.
My own helmet use has been spotty. While discussing this issue with some friends I remembered that when I lived in Munich I didn't wear a helmet. In fact I don't think I wore a cycling helmet at all until 2000 even though I have been cycling regularly since 1975. After I bought my first helmets in Oregon when I moved back to the US I wore it 90% of the time. Though I refuse to wear a helmet while doing maintenance on my bike and test riding it in a parking lot or around my block.
Even when rock climbing I haven't been 100% on helmet use. I used my helmet mostly when climbing longer trad climbs or even three pitch trad climbs but for sport climbing I often didn't wear a helmet. In fact when climbing I often found I bumped my head quite a bit more when wearing the helmet.
I have to admit that though I do wear my helmet regularly now, if I didn't have too, I probably wouldn't. It is very convenient not to have to carry this ungainly artifact. And I like to use my bikes for getting around, not just for events, charity rides and racing. And the arguments you often hear about how little you value your brain are easily disarmed when you realize the person talking to you isn't wearing a helmet while engaged in the dangerous activity of going down stairs, crossing the street, or driving their car. All of which have higher fatality rates then cycling measured as deaths/hour of activity.