C/L Stunters also get judged on looks and flaps that blend smoothly into the airfoil help shed the "toy plane" image. Having the flap slightly slimmer at the hinge line is supposed to contribute to smoothness. Minor movements of the thinner flap don't get into the airstream, so the plane is supposed to groove better. This idea was brought up to me in a book written by Dick Mathis and I've used it since then with no complaints.

I can't think of any full scale examples of planes [that fly at a decent clip] with control surfaces that stick out above and below what they are hinged to. It doesn't even look "right" to the untrained eye. I don't even think the design engineers at Blum und Voss would disagree here.