Actually, I would say that neither knock-knees nor bow-legs are desirable, but they're very common with inexperienced riders (or more experienced riders who've never been taught how to pedal correctly).

Typically women are more knock-kneed; men are more bowlegged. The condition has to do with hip flexibility. Neither is an efficient way to ride because you see wasted motion in the pedal stroke. It takes time to correct the flexibility differences and track in a straight plane. While we don't want to "force" the pedal stroke (which causes stress on the ankles, knees, and hipe), we want to correct this within an acceptable range of motion for each individual.