I don't understand the point of the various tank/dps stances for tanks

[Jim_Berry]

So far the way it feels now (just kickin' around, since coming back, leveling in Stormblood), tank stance is irrelevant and unnecessary for anything other than a sort of arbitrary difficulty switch you can either turn on (to make it easier) or turn off (to make it only slightly "harder").

I not dps-savvy on my tanks, but I WILL say this:

You must not know DRK that well, because our strongest potency attack (at 650) comes from being in Grit + Dark Arts (it's name is Bloodspiller).