I suppose the answer would be the same as the one Squatter put before: the Morgul-blades were probably very rare weapons and not to be used just for stabbing around. Don't forget the blade broke when it hurt Frodo; the possibility of losing one such weapon in exchange for one little wraith (after a little time of waiting) is not much of a win. Also, Sauron wouldn't want to send some sort of newly recruited wraith on such an important mission; we know that he wanted to send only the Ringwraith because he was 100% sure of their loyalty: they had "no other will than his own" and they were bound
by the Rings. Also, when Gandalf explains to Frodo what would've happened to him if he was not healed, he says that he'd become a
lesser shadow under the command of the Wraiths. So not much of a gamble as well. One more less powerful wraith won't change the odds. That's to say if Sauron even had as many horses
