There can be a number of reasons why the Mantle isn't on top
anymore, one of the most obvious reasons is that it doesn't stack with Dark Orb/Avatar Symbol anymore which basically means that the +6 base you get from set shoulders >> Mantle aslong as you own either of those 2 other items. And ofcourse if your tank really needs the 10% extra base for better aggro management (if you don't have chestpiece yet), if this is the case find a new tank!
If you read up/back a bit you will realize why Revel put the Coercive pants back on top. Practically no guild uses 3 healers in MT throughout a zone, which means MT group will prolly look like this: Shammy - Temp - Tank - Dirge - Scout (can't really be specific but my guild runs with a Swashy here) - You. With the pants on you give extra MC to those toons which will benefit the raid
more than Gynok DPS pants.