The "core" run for my guild is made of a disc priest, holy paladin, and a druid. We rely HEAVILY on our disc priest for bubbles, and to pick up the heals that might go unnoticed or gaps in between. I think disc priest, and holy, are wonderful I have a holy/disc priest myself that's been in ICC.
Though I do think on my server seeing them in PuGs is a lot less common, but there aren't a lot of priest on my server.