I hadn't looked at toot's link, Lonelier_Planet.
There is certainly a difference between someone who can play Send in the Clowns on the clarinet and someone like me who would know which end of a clarinet to put in my mouth but no more than that. It's a difference that we might want to describe in words sometime. We might decide that a way to describe the difference was to say that I can't play the clarinet while the SitCs guy can. It doesn't really seem unreasonable to me.