Rapture really sucks. Fights can't ever be designed to require a health buffer since only Discs can provide it, so fights won't ever be designed to require Rapture, and PWS spamming isn't a great way to apply atonements.
If Rapture either reduced or eliminated the mana cost of PWS during it's duration it would be a lot more valuable.
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The power of Shadow Covenant threatens the viability of the "correct" way to play Disc, through atonement healing. Grace is for dungeons to help with tank healing.
CoW is worse in Legion than in WoD due to Disc's largely reduced mobility. Right now we spam PWS and can move however we like during that, so taking time-out to stand in place for 2 seconds isn't a big deal. We'll have little opportunity to move in Legion raids, so it's questionable to stand there casting a spell for 2 seconds which has such a mild throughput value. Only Power Word: Radiance has the same long cast-time/mobility issue as CoW, but it's far superior on throughput.
Despite the popularity on these forums for Purge the Wicked in Legion raids, we'll have to see once Legion goes live how it compares to Shadow Covenant.