Look at the gap and think of it as a leak in your wing. Sealing it gives you more response from your throws, no question. Also it decreases your wing loading since your wing area is effectively bigger. Adds resistance to the servo? Only if you do it wrong. Take a piece of ultra coat in your hands and bend it back and forth. It takes next to zero torque. If you apply the covering without giving the hinge enough slack to deflect, then of course it will bind.

AND... it's so easy to do. Why not?