It seems like it could be a useful tool, but apparently is being underused. One possibility for breeders and their clients to address the issues brought up by OP would be to create a listings for themselves (if professional) and rate others they have had dealings with. Then, on their websites, in signature phrases and other promo, say "Listed on ratemyhorsepro.com". It could be a little like when you check seller's feedback on Amazon or Ebay.
I don't think this is a perfect solution, but it could be one way to go, when someone knows a breeder is bad news and doesn't want to post here or other places for concern about legal or personal fallout.
BTW I have absolutely nothing to do with that site and have no idea who does.