These have been in use for several decades. As GAgun said, they are used on skeet and trap ranges, especially during competitions, to rest the muzzle of an open breach loader on between shots. They are perfectly safe and I'm unaware of any accidents associated with them.
Yep. See them all the time and I still get the creeps when someone uses them. I break action and rest it over my shoulder, cant stand a barrel pointed at me regardless.