I think that based on the Legion changes Blizzard should solidify the roles of the healers. Paladins have always been the tank healers, and every other healer sort of jockeys for position as raid healer and thus the only thing that sets them apart is the utility they bring.
I personally think that in light of our new mastery that Resto Sham/Druid would be the kings of raid healing, that Pally/MW would be kings of single target spot and tank healing, and that Holy Priests could talent to focus more on tank healing or raid healing but not both at the same time, with Disc being 50/50 atonement healers.
Ultimately MW lacks a niche and something we're good at that makes raid leaders want to bring us.