hogflyer covered how useful it is in flight. I'd like to add to that just one thing. Have you ever seen a modeler fly pattern maneuvers? Or seen video of the guys who fly at meets like the Tournament of Champions? You don't have to be that good to use the rudder, but you won't be good at all doing lots of maneuvers unless you learn to use the rudder to correct the airplane's path in maneuvers.

One of the first maneuvers most developing pilots learn is the cuban-8. One of the flying fields around here has an "expert" who claims the rudder is worthless. Some days his cuban-8s look like he's only been flying for a week. He's been flying for years and years and years. And thinks it would have been a waste of time to learn to use the rudder.