Have there been any report of broken or damaged mount (lens and/or camera) and under which conditions?
Yes, there are lots of reports of broken mounts and flanges, even right on this site. The conditions under which the damage occurred aren't always explained, but dropping the camera with a lens attached seems to be a frequent cause. That shouldn't be a surprise, as the force at impact would be many times the force that would normally be applied to the camera. Examples:
The lens mounted on a tripod can support the weight of the camera, but likely not vice-versa
There are other reasons beside protecting the mounting ring to support the camera and lens near the combined center of mass:
Reduces load on other parts of the camera: Unless the lens mount breaks, all the force that a heavy lens places on the mount will be carried by the camera body and the tripod mount.
Makes the tripod much more stable: Tripods are most stable when the load is centered; if the camera's center of mass is too far from center, a tripod becomes easy to knock over, or might even tip on its own.
Makes the tripod head much easier to use: Tripod heads aren't meant to stand up to large forces, and they generally work best and are nicest to use with the load roughly centered. It's no fun when the camera drift downward because it can't handle the load, or to have to crank up the friction to fight gravity.
So yes, it's a good idea to use a tripod mounting ring with large lenses. I don't have the data to know whether the mounting ring on the body, the flange on the lens, the body itself, or the tripod mount on the body is the weakest link, but it stands to reason that if you apply enough force, one of those is going to have to give out. Whichever one it is, the outcome is pretty much equally bad. Using a tripod mounting ring protects all of those parts, and makes a fall less likely, and makes the tripod easier to use.
