Conditional mitigation has no gameplay value unless it is connected to a player decision (i.e. giving the player the chance to swap from physical only to magic only based off of an animation tell on a tank buster). In its current form it just ends up being a limitation that arbitrarily puts DRK at a mitigation disadvantage at the whims of the fight designers.

I'm curious about whether the move to a 15 second Reprisal/Feint/Addle represents a shift in raid design that could benefit Dark Missionary/Heart of Light's longer durations, but I'm not holding my breath without direct nerfs to both Shake it Off and Divine Veil. I think they might have their ducks in a row by the time that the 8.x reworks roll around and the next tank job is ready to be released, but my gut feeling is that it's probably safer to give DRK a hard pass for Dawntrail.

Tank design is far too unbalanced in favor of WAR right now, and I haven't seen much to suggest that this is going to change anytime soon.