Why do people use prey animals as guardians?
Because some prey animals are fiercely protective of their territory, and more than capable of defending it against some predators (just ask the person whose pet dog got killed by a single kick from a donkey!) They aren't useful in every situation, and some aren't useful at all (being just as much a danger to the animals you hope they'll protect). A lot of donkeys bray if they see anything unusual, thus acting more like an alarm system. Depending on the predator, LGD's can be overmatched, too; I firmly believe that a person's predator protection should primarily rely on fencing/housing that is highly predator resistant.