The assumption is that wearing a helmet somehow reduces the odds of exercise. Just keep your helmet with your bike, and you'll never forget it.

I've been cycling to work for years, which which has varied from 10 miles one way, to 1 mile. I always wear my helmet, and in all these years an many thousands of bike miles, it only been "useful" once. A drainage grate was covered over with leaves, I didn't see it, my front wheel fell in, fork snapped off, and I flew over the handlebars, headfirst into the curb. My helmet split in two and I had a massive head bruise, but had I not been wearing it, I'd be dead.