True AB, people do change trainers a lot. I don't think asking for references from others at the same riding level is a bad thing and generally those that did move on would be a good gauge of progression & how things ran in general without feeling like they have to talk up their trainer.
I have a lot of people ask for references. I always ask permission from current/past students before giving their info to a prospective client however. I even include one that the relationship didn't end so well (pushy mother that made her daughter fall apart when she insisted she do something she wasn't ready for...she made her quit because she was too scared to try. It was horrible) I think for me it's kind of a norm because I do both training & instruction. I expect people to want to check me out thoroughly and make sure it's a good match for both of us.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.