I imagine external seals are much more critical/effective on high-performance gliders where transition to turbulent flow occurs aft of the aileron and flap hinge lines. Otherwise a nice gap could certainly cause earlier transition.

Typically, on older gliders where transition occurs ahead of the hinge line the manufacturer recommends Tesa tape on the bottom hinge only. Having a smooth/flush seal isn't so critical anymore since the flow is already turbulent. However, it's still effective because it's a pressure barrier and prevents flow through the control gap.