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Actually, both ailerons don't have full effectiveness. They cause the yaw because they aren't equally effective, one is creating much more lift and drag (they come together as a combined "gift") than the other. Their imbalance causes the yaw in the first place.
If an aileron is moving 10 degrees not 30 degrees then it does not have the full effectiveness that it could have. I was referring to the effectiveness it does not have from reduced deflection not the aerodynamic effectiveness it already lost. Besides the roll rate is reduced mostly because of the side loads from the yaw and the interaction of a wing with dihedral. Wings with no dihedral do not seem to significantly slow down their roll rate, even if the yaw is enough to see the tail wobble.
The goal has always been to rig a full scale to be as efficient as possible and to take as little trim as needed. Less surface movement = more efficiency (and a cleaner flight). Differential aileron (when needed) has always resulted in a more efficient roll. Always given less drag for more roll from less stick needed.
Adding in yaw does add drag, but not enough to matter. I am flying a model already getting very poor efficiency due to its nitro engine, I couldn't give a flip. Besides, as anyone who has flown a full scale plane would know it actually flies very well that way. In fact you may flunk your flight exam if you don't add rudder in the turns, even on aircraft such as a Cessna 150 which has differential. The fact is my planes have a slightly faster roll rate with no differential and rudder than with differential and no rudder.