Ruddering is one method of turning, it is not the most ideal and has drawbacks on the amount of control you have etc. However there are times and places for all types of turns. When all people can do is rudder that is when it an issue because it will cause them to be unable to advance.
It is no different then skiers using the little hop turn thinger before they learn how to really carve. It lets you get away with bad technique but will bit you when you get to more challenging terrain.
Anyway everything I just typed is conjecture because I don't have any certifications etc. It is just how I view things.
Also I think most people rudder when they are not confident in their abilities on a run because it is how many people first learn to link turns. (reverting back to basics)