Well, assume mules and hinnies are functionally infertile. Breeding is zero reward, but not zero risk. As well as the risk of injury, there is the risk that a male mule mating with a fertile female will result in a miscarriage of an unviable foetus rather than the young you actually want. There is also a smaller risk than a female mule mating with a fertile male will cause him to be unable to successfully stud for the fertile female you want to breed him with, although this is a lesser concern.