I rode horses for other people last summer, and generally only ride horses I raise and break myself. I grew to realize how spoiled I had become with my horses very easy obedience and was so burned out at the end of 3 month/15 horses, I went back to riding just mine, which I don't even do as often as I'd like.
It is amazing what horses with beginner riders will do. They get away with something, then go about their merry way and do whatever they want, with the poor beginners trying to cope with a "beginner friendly" horse! Half the horses I got in the ride were from beginners whose story was always the same - they get the horse, everything is perfect, then the longer they have it, the more problems pop up until they can't ride the horse at all.
This doesn't really go with the thread of getting a horse above you, but, depending where you are in your riding, some people need to take a few more lessons before jumping right in and getting a horse they have no idea to control. It could lead to a good horse having some not so nice qualities.