Control of Yaw is IMO consistantly the weakest part of flying skills in the PPL's I have flown with. This is because unlike classic trainers (cub,champ,C140 etc) you can get away with flying modern trainers with your feet flat on the floor as much of the Yaw inducing factors have been designed out of the airplane. With respect to teaching turns in a C 172 or Pa 28, the adverse yaw is subtle enough it is not reasonable to have a student be able to detect and correct for it by visual indications and seat of the pants feel. Therfore you have to use the ball as an aid for the student. Eventually the student will automatically apply the appropriate rudder, and be come less dependent on using the ball to correct yaw. But I strongly believe this must be taught right from the beginning. An emphasis on maintaining coordinated flight that is established from the very first flight will build life long good habits.