I don't do 25 man, so there could be a massive difference, but you know those times when you need to cast a Heal on a player because he's damaged, but the rest of his group isn't? And you need to prioritize the PoH on the other group and then go heal him. Or wait to see if he can make it long enough until the rest of the group takes damage? Well, by making PoH smart you've eliminated that entire thought-process .
Yeah, Holy Priests are perfect, too bad the raid design isn't.