Well, they both have their purpose. Most of it depends on the plane you're flying. I know there are some that will only use the rudder on a plane with dihedral wings. this is not advisable on sport and 3d planes. The rudder helps keep the plane in it's track durring a turn unstead of fighting with the elevator and ailerons. Up until recently, I was like you and only used the rudder for ground control. On most planes you can get away without using the rudder in the air, but for aerobatic and 3d maneuvers, it's almost a necessity. It's not going to hurt to start using the rudder. It would be helpfull in a crisis like if an aileron servo gets stripped or in some cases to slow the plane down. if you're flying a trainer or something with a dihedral wing, try using only the rudder to turn in the air (make sure you're a couple mistakes high). After a while, it will become second nature.