I just don't really see a reason to bring any other healer.
Resto druid brings combat ress, good dps, solid healing in combat and outside of the combat. utility in form of roots, they can stealth, have on demand damage reduction in bear form and can deal with all affixes.
Now to compare it with discipline priest, disc priest brings good group and st damage reduction, have solid party healing and do good damage. But on the other hand, their single target healing is terrible and their healing outside of combat is beyond crap. Dealing with grevious is nightmare and bursting is quite bad as well. So idk, maybe if they nerf some and buff some and stuff will happen, I can see holy paladin or mw monk replacing resto druid, considering you bring blood dk or some sort of combat ress