everything I've read about flying indicates that slip landings are practiced, because when you need to do it you better know how to do it to avoid becoming a smoking crater.
Opposite rudder is used to lose height quickly. You put the plane on its side, and then push yourself down using the rudder to point the flow of air upwards. This results in sliding sideways downwards very quickly... and there are times when this maneouver is the difference between life and death; for example, when one is close to overshooting the runway.