Differential is used for two reasons. One to reduce adverse yaw created by the downward aileron producing more drag then the aileron moving up. The second is to counteract the the aileron moving down will result in more gained lift then the wing with the aileron moving up has lost. So by introducing differential you fix both situations. I do agree that for full scale it's not an issue as one can feel the slip and simply lean on the rudder. Our models are a little different as for competition aerobatic pilots we strive to get our airplanes to fly as pure as possible. This means getting rid of as much control cross coupling as possible. Everything is a compromise, while aileron differential will help rolls on the verticals and upright it does do the opposite while inverted. Most guys find that more manageable then having the nose wander on up line rolls. Imagine trying to do 1/2 of an 8 point roll followed by an opposite 3/4 roll on an up line and pulling to an outbound cross box travel 90 degrees to the runway when the airplane is not rolling axially. This can be farther proven by different setups required when ailerons are hinged at center Vs. top hinged on some all composite models. The center hinged requires that through the TX you set positive differential. On a top skin hinge aileron your TX would be set to negative differential because the mechanics of the top hinge actually has you ending up with too much differential.