These are extremely rare events. It would be like wearing a helmet in case your windshield fell out. Having some in the cockpit for emergencies could help, but I don't know if it would be possible to put them on while handling said emergency.
Helmets can help race car drivers in crashes, if you're in a plane crash it's unlikely to help much. Fighter jet pilots wear helmets mainly for breathing at high altitudes (that most GA aircraft are incapable of flying at), as well as for things like an HMD.