I think that recent changes to the law would actually give breeders a pretty good chance of getting a dog back if a purchaser breached the agreement - but breeders would have had to been to see a lawyer very recently to get details of how to do it. It means taking things a step further than just the contract though.
I think that the contracts are more for a breeder's peace of mind, rather than being actually enforceable, if the contracts I've seen are anything to go by.