Why do they want to use the dog, is it a particualry good specimen, do theblood lines tie up with the bitch, is it a top show winner, is the breed short of stud dogs - there are many questions that need answering and considering before letting your dog be used just because someone has a bitch of the same (presumably) breed. Once used a dog can start to lift his leg indoors, hump cushions, chair legs, human legs, kids, and want to stray to find bitches. His temperament can change (mostly not for the better) and also the stud dog owner has a responsibility for bringing more dogs into the world - can the owner be sure the bitches owner is responsible enough to find them all good homes and take them all back if things don't work out once the puppies have been sold.