I'm still failing to see at all why you wouldn't maximize your GCD use and stance-dancing if a fight required full Fending for some reason (such as for tank busters more akin to those from ARR, wherein it was much more difficult to run full Slaying). Nothing about the playstyle has to change even if they start requiring more HP from tanks to survive in future raid content. Tanks are still going to want to push as much DPS as they can, especially when tackling content at lower gear levels, where DPS checks are harder to meet.