Yeah Holy has been above and beyond every other spec on fights with constant aoe damage this whole expansion: I used to run as holy on Garalon, Tsulong and Garajal aswell, topping the meters without any effort.
But the number of fights where holy>disc is so low and the stat priority is so different that you will hardly see a priest switching from one to the other spec based on the fight.... Unless you're a finnish guy running for a world first :V