Let me put it this way.
Why do we have DD tanks in the first place? To deal damage while tanking. If we wanted 100% most survivability, there wouldn't be any tanks but PLD or fully eva geared evasion jobs that output little to no damage at all.
DDs are put on there because even though they're less survivable, you can still keep them alive.
If you find a mob where the 10% counter is going to be the diff between consistently winning a fight and consistently losing a fight, then you'll have a point on Spharai, but until then it's just what PLD tanks were. A saftey net. It has better survivability, but if you can survive with both, why not use the one that does more damage?
