Maybe we have to ask a different question.
Are we talking about "back yard breeding" where an "oh, so beautiful stallion" is being stood to an "oh, so special mare"? Without any regard to type, color, movement, etc.? If so, it's a monumentally bad idea.
Are we talking about trying to create a new type, and ultimately a new breed? If the breeder has the time (several decades), money (a lot), and expertise (including a good eye for brood stock selection, foal evaluation, and a willingness to put down breeding failures) then I've no real heartburn with it.