Excess knee action and a short, choppy gait not only wear down the horse, but also the poor rider who must keep posting, posting, posting to each beat of the trot. Second, a good moving horse, in general, is a sound horse. There is less wear and tear on the joints and tendons. Finally, a good mover adds eye appeal to an already winning hunter round. There is no high-stepping action to detract from the horse's approach to the fences or its jumping style.
Judges know precisely what they're looking for in a good mover.
Andrea Meek has been a licensed judge with the American Horse Shows Association since 1989, and she explained how she evaluates the movement of horses in a hunter under saddle class. I consider a good mover to be naturally balanced, with a flowing, even stride. While lots of horses appear to be good movers from the side, flaws can be seen if the horse is observed from different angles. Such is the case with horses that paddle, wing, or habitually travel with their haunches off the track. In the bigger classes, it can be hard to spot such deviations, so I try to stand near the corner of the arena, as long as I'm not going to get run over.