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.

Also, mixing rudder to fight the unwanted yaw adds even more excess drag to the already screwed up airplane. The poor bird had ailerons that can't create clean roll and you're throwing rudder and it's increased drag (it's gotta create lift to work and that combined gift shows up again) just throws gasoline on the fire. You wind up with a doubly inefficient configuration with both the ailerons and the rudder being needed when just a touch of aileron and some elevator could do the job efficiently.

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.