My Mirage sailboat has this same treatment on both the rudder and fin keel. I suspect it allows a stronger trailing edge with minimal increase in drag and less possibility for damage when backing up. Lest we forget, flutter causes drag, also! My Funtana has tapered ailerons with approximately a 3/32nd flat at the T.E. The rudder and elevator are 1/4th thick balsa. They are flat with no taper. The L.E. is rounded, the T.E.s are not. The airplane is designed for 3D manouvers and slower speeds. I'm not about to peel back the covering and re-shape the T.E.s, based on input from this thread. I'm going to go with the designers knowing what they're doing. After all, they are famous for their abilities and knowledge about design and who am I, to second guess their designs.