We don't have enough evidence to say that it's either common or uncommon. When I asked people to check their own cars, no one did. We only have 3-4 confirmed cases within the forum because there have only been 3-4 people that are actually aware of what the underside of their own car looks like. My car was leaking, but it drove great, shifted great, and had no symptoms.
I was going to start a poll asking people to post their mileage along with a picture of the underside of their bell housing; but, after seeing that no one cares enough to check their own car, and the fact that I had given up on my own, I figured it was pointless.
I'm actually not surprised that few people have checked their own car for this oil leak. Inspecting it involves removing both plastic shields under the car and safely raising the car to see underneath. If you're not doing your own oil changes it's a lot of work to inspect this yourself. Ideally it should be checked out by whomever is doing oil changes but I wonder how many dealers will just wipe the oil off the transmission with a rag and call it a day.