I wonder why "most serious trainers' have to do this?
The serious trainer is trying to keep a horse tuned up for an amateur or youth who does not not get the respect or response that is needed to keep winning (or at least riding at a higher level). So, the horses usually goes downhill -- some very fast and some gradually. Obviously the owner/rider is not riding at that high a level but they want to win or at least make a good showing. Most are not going to do it on their own. That is why they need a trainer and/or a coach to start with. They want to ride above the level they are capable of training a horse for. They want to buy a winning 'finished' and then they want him to stay that good and not go south.
Ray Hunt would say don't ruin that horse's desire to help you, you take away his interest in doing what you want him to do by drilling him, training on him.
Actually just the opposite is true. You 'hammer' the mediocrity out of him and he gladly responds to the light aids you use to replace the hated nagging. If you do it right, and follow up with consistent use of light aids, only reminding him occasionally that you won't accept less, he will stay happy and responsive. The only reward a horse needs is a lack of punishment. The worst punishment you can do to him is pecking and nagging at him.
If you want a sour, sullen unwilling horse, just demand very little and constantly nag and peck at him. You will kill ALL try and get nothing better than what you have been accepting.
When you get after a horse that really knows better and get after him hard and immediately set him back up to ask again, with a very light touch, you have told him in a way he understands that you want the best that he can give. A 'good' horse that is not being asked to do something that is beyond his capabilities will cheer right up and try to give you the right response. You will see a better attitude, a light response and a MUCH happier horse. Nothing destroys an attitude worse than constant nagging and pecking.
This is a very advanced training concept. It is why I have not explained it earlier because most people who have not trained or at least ridden high level performance horses do not understand how it works and probably get in trouble trying it because it is way over the average rider's capability.
When I work with riders aspiring to get to big shows or ride competitively at a high level I try to stress that riding at a high level and training at a high level is about 60% mental ability and mind-set and about 40% riding ability. You ride and train at a high level with your head. Your hands and legs just follow through with your mental mind-set.