Rudder is good for scale flying ... try turning say LEFT with your rudder, of course you will use the ELEVATOR to compensate for the loss in altitude, now try to use a little OPPOSITE AILERONS ... this will make your plane turn to the left but the wings are level.

RUDDER is for YAW ... means the NOSE of the plane turns unlike ailerons where the plane rolls a bit (wing drops) and you use the ELEVATOR to make the plane change its direction.

I learnt to use rudder more recently and like stated above you need it for Knife Edge, 4 point rolls, slow rolls, rolling circles, hammer heads/stall turns and even to correct flight on a vertical. I know now how important it is to use the rudder especially when you land in a cross wind!

Only Q500 pylon racers do not need rudder ... hehehe ... they are a confused lot ... always flying in circles only!!!