Originally Posted by calcapt

Steve:

Airplanes could turn using only rudder much like a boat in the water; however, an airplane moves around three axis' and the comfort of the passengers and efficiency of operation requires coordination of all three - Roll, Pitch and Yaw. If you were to turn only using rudder and no aileron the passengers in the back would feel like they were being swung on the end of a rope and other undesirable characteristics would also present themselves but the story gets too long to tell. Suffice to say that the present system provides the most comfort to passengers and has less of a chance of spilling the captains coffee. Good question though!