Gotta admit that that made me go "hmmmm."....but yeah - the drag, turbulance on the primary sail, and lift in the wrong direction when healing would be substantial drawbacks. However, if you took this idea and established it out far away from the mainsail and made sure that the lift generated was always to windward, or leeward depending on point of sail, you would end up with the kite sail that kite surfers are using (and that are just starting to see use on sailboats).