It kind of works like seatbelts. You can't prove that the fact someone fastened seatbelts had saved one from death in the accident that already happened. Some people survive, some people die. But the relative proportion of those two cases is the matter of scientific studies - that's the first method we have to determine that seatbelts help save lives. Another one is a mix of physics and common sense. It's better not to be ejected from your seat at high velocity and fly through glass. In the same way, it's better when the screen does not hit the ground (as rubber cases are bigger than the phone and thus protect from impact) or hits, but with much smaller velocity (as rubber absorbs the energy from collision with the ground).

So yes, we can't say much from all the anecdotes here. But there are strong reasons to suspect that phone cases are a net win in protecting screens from damage.