I think it depends on the stallion. Some studs can handle being around mares and not covering them, while others can not. Then again, the Bruce effect is well documented (where a female of a given species aborts/resorbs when introduced to a new male). It can be a matter of management/training but at the end of the day is a liability to co-house studs and other horses. If a person feels that its worth the risk than more power to em. That said, others feel its not worth the risk and that's also perfectly acceptable in my book. If I had a stud I would house it with mares or geldings for the pure reason that a stallion with his fighting dander up is not something I want to handle.