People already whine about holy pally gear; how much more will everyone complain when you have 'tank' mail and, far worse, 'tank'/avoidance cloth.
Or, there's the opposite gear argument: things get so awkward when they awkwardly force specs into existing gear. Spirit for spriest +hit? Very unpopular with healers. Same thing for resto and elemental shammys. There have even been attempts to make strength the spell power stat for hpallys, which have understandably failed thus far but with every attempt they get closer to accepting it as a solution. Imagine, just IMAGINE, how weird things would get if, say, a shaman got parry from crit, dodge from haste, and damage reduction of some sort from mastery. And no, it wouldn't work to simply pile it all onto mastery; Blizz actively discourages a single secondary stat grossly outperforming the others.
So yeah, for this to work we're either gonna see +dodge mail or some strange crit --> parry solution. Either one is an abomination. It just won't work at this point. Maybe they'll get it to work for WoW2.