There is a lot of truth in what Core6430 is saying. A horse with fancy gaits will come in the ring with a potential for higher scores. A rider on a horse with a gait score of 8 is going to be at an advantage over a rider on a horse who might score 7 or 6 on gaits. Make a small mistake on a horse with a gait score of 8 and that person might still score better than a rider who is on a 6 or 7 mover who rides correctly.
A horse that doesn’t score as well as a fancy mover isn’t necessarily a “crappy horse”. It might just not be purpose bred for dressage.
I also think there IS a definitely issue with trainers who advise their clients to buy horses that end up being “too much horse” for them, such that the trainer needs to put in a lot of training rides. I know several people personally who ended up in that situation, i.e. letting themselves getting talked into a certain horse being perfect for them to progress, but then they can’t ride it! Clients who want to ride and improve need an appropriate horse, not a horse who ends up being a power grab for the trainer. If someone wants to buy a horse specifically for a trainer to ride, that is fine, as long as that is what the client wants.
Also, Core’s comments are also supported by the fact that there are rider tests at each level, something that focuses more on the rider’s ability to get the job done more so than the horse’s gaits.