I have been using Shadow Covenant exclusively for awhile now. So far I can say that if used correctly atonement healing isn't really necessary anymore. It honestly simplifies many things. It can allow a disc priest to be reactionary much easier than with atonement--i.e. adaptability is increased.
Look up Ahili-Eldre'Thalas in warcraft logs. I cannot have a link in here cause I am new. There will be some logs that include the use of Shadow Covenant for raiding.
Emerald Nightmare Normal and Heroic are really the only ones populated. These are all pug groups.
Feel free to analyze the data and consider it. Perhaps it will give some data to a much heated debate.