That's because Holy Priest and Monks are probably the highest output healers right now. Unfortunate, Resto Druids come no where close to the amount of sustained and burst healing a Holy Priest can bring.
There's no real advantage to having 2 Discs in the raid for most fights this tier (or even 1 Disc for the latter half of ToT, barring Dark Animus), and your other Priest will be much better off as Holy.
That entirely depends on your strat. I've tried to put out feelers about going Holy, but most of the time they want Pain Suppression, Barrier, and Spirit Shell, not extra healing. And a Druid brings Ironbark, which isn't Guardian Spirit.
Still, the Atonement nerf isn't entirely unjustified as it does too much healing and damage for very little effort. Compensation in the form of modifying the T15 bonuses to benefit Disc more or by reverting IF to its older state of double healing + guaranteed DA would be nice though.
Doubt we'll see that.