The person you're quoting is also kind of wrong. Unless you can multi-DoT like a crazy person and/or don't have to move at all SPriests are very weak. If you look at logs for ABT, outside of Coven and Felhounds which are the one fight type Shadow excels at, Spriest is middle of the road at best. And that includes massively cheesing damage on the small adds on Aggramar.