And not just for backup. I switch back and forth between single and a greenland blade. I enjoy the feel of the single blade, but then, I think I am a canoeist at heart.
I'm 6'4", so maybe I can get away with a few inches more paddle than shorter-torso'd paddlers, or maybe my paddle is too long and I just don't know any better. 46" sounds about right.
I switch sides about every five strokes. The switch would be easier with a shorter paddle. Also, if you have a rudder (I don't), it might be easier to just stay on one side.