Don't confuse "efficiency" with fuel consumption.

The reason designers rig differential into their airplane's ailerons is to make the sucker fly better and safer. Better can be a synonym for efficient. Matter of fact, in this case it is.

An airplane coming in for a landing that has the ailerons cranked into the cross wind and then needs rudder to kill the adverse yaw and resulting slip/skid has a number of things going against it. Roughly speaking, it's envelope has been reduced in a number of things by all that's wrong with it. It's not a very efficient design in certain areas and is generate excessive drag to fly the pilot's chosen course. Solve some of those problems and the airplane flies more efficiently.

For example, move the CG from an overly safe forward location, and the elevator needs less deflection to give you the pitch you want from it AND the pitch stability isn't affected. That elevator works more efficiently. If you understand what you've done by moving that CG to a "better" location you'll know to use less stick or re-rig the elevator connection, and the airplane will fly more efficiently. It'll actually be able to fly a little slower and do it with better control. It'll also lose less speed from pitch changes. Why, because of the increased efficiency. What happens to the gas mileage? Who cares. And make the ailerons roll the plane with more efficiency and you'll need less rudder to make your turn more efficiently. Efficiency just washes over everything from everywhere when you tune just one thing to suit the plane better.