It seems that, according to evidence from the top raiders currently, DRK is the preferred tank for magic fights. A1 Savage had the 25k magic nuke and I'm imagining A4 is also going to be done with DRK. A2 was done with PLD most likely because its a bunch of adds with physical damage so Blocking and Physical mitigation is king there. No idea on A3 Savage.

My guess is that "highest DPS while MT" along with magic mitigation puts it over the top on these DPS -check heavy, magic fights.