In a very real sense I think of all healers as specialists rather than shoe-in deploys. With few exceptions healers neither block or clear, so typically the right answer to "How many healers do I want in my deployment?" boils down to "As few as I think I can get away with." Whether that means slotting Feng Shui or Priests depends entirely on the map given that they're both strictly better in different circumstances. I understand that's a wishy washy answer, but it's honestly how I think of these things.