It does seem like something that would rarely be used. I mean you would have to first see that a unit needs to be healed (and since it would cost a sheep/cattle it would only really be relevant for heroes and possibly elephants), then find a priest that is not busy, then find a sheep/cattle, then find the unit again. If the unit would still survive that is since going back and forth would take a fair amount of time. To make the most of it you would also only use it when the unit is just about to die (otherwise you would have spent the animal just to get back a few hitpoints), so you would have to spend time waiting. All-in-all, nice idea in theory, but I doubt it's going to be useful in practice.