When I started racing mountainbikes downhill, only the Pros used to wear helmets - and the rules then switched to make it mandatory.
As students, we couldn't really afford bike helmets, but Giro brought out a crash replacement policy: Smash your helmet and they'd replace it for half price. I stumped up the cash (my girlfriend at the time was insisting it was a good idea) and forked out for a comfy "peanut" helmet. That afternoon the club went out to practice, and I grumbled all the way on the ride to the DH course... once at the course we started running practice runs, and I fudged a big double. I missed the landing spot by a few inches, and the front tyre hit a wet tree-root. The wheel spat out from underneath me and I thudded into the ground with the grace of hippo.
I saw stars for a bit... and felt damned groggy, but the bike was ok.
My brand-spanking new helmet however was worse for wear. It had become 3 pieces, and had a golf-ball sized sharp stone lodged in it. If that helmet hadn't been on my head, the stone would have hit my just above the right temple, and I may not have been able to sit up and whine about breaking a brand new helmet.
I haven't questioned it since then.
If someone can't see the benefits of a helmet, let them be: There's probably nothing worth protecting up there anyway