This is meant to be slightly humorous and tongue-in-cheek.
While I think every horse benefits from lower level dressage, I am not sure I would ever want to teach very hot horses how to do upper level movements. Riding hot horses out on the trail, we have seen some very interesting things happen when they get too excited, but yet are well-trained enough to be controlled from galloping off. For instance, if one of our horses has to be held back to a canter at a spot where we usually gallop, she will start doing one-tempi flying changes over and over. This is not very fun if you are on a narrow trail or on rough ground. If another horse is held back from the others at the trot, his motion gets shorter and he starts getting more and more collected. The farther the other horses get in front of him, the more collected he gets until he is doing passage. Which means eventually you have to shout ahead for everyone else to wait up or you will never catch them. Another mare throws in the capriole as a way to gallop off without a person being able to stop her. She seems to be in a controlled trot, then shortens in a split second and next thing you know you are flying through the air. With no ground under your horse's legs, it is impossible to keep her from thrusting her back legs underneath her as she springs off into the gallop. She is learning not to do this by having the rider put a bend through her body when she feels too collected - she can't leap off when she is bent.
So to me, the more movements a horse knows, the more crazy evasions will pop up when your horse gets over-excited on the trail. I shudder to think what would happen if the mare who caprioles knew also how to courbette and pirouette at the canter.
I guess it proves that what they say is true; every dressage movement is something a horse will do on its own naturally at times.