Mountain bike riders aren't legally required to wear a protective helmet. But if you talk to anyone who has ever had a bike injury while wearing a helmet, you'll hear enough to convince you to wear a helmet. It just makes good, common sense.
There's no guarantee that taking a spill from a bike will result in a quick death. A helmetless rider can sustain serious injury and damage to any part of the body but especially to the face, head, neck, and shoulders.
Helmets protect against permanent brain damage in those who survive a fall head first. Most mountain biking accidents do involve falling forward over the handlebars. Often the rider is going downhill at high speed.
The helmet doesn't protect the very mobile cervical spine. There are numerous cases of neck fracture and permanent paralysis reported. Most of these accidents could have been prevented with proper gear, paying attention, and reducing speed on uneven or unfamiliar terrain.
A helmet is a small price to pay for a large amount of protection. It just makes good sense.