I'm the Warrior she's referring to. The only times I drop Defiance is for fights (and by fights I mean Primals or weaker dungeons) in which there isn't hard hitting, unavoidable moves back to back. Having 8k base HP before Defiance means that I can eat a first MB, then pop Defiance/ToB before the next, wait for cures, turn it off and begin off tanking with Berserk. Swap in, pop defiance, BB a few times, repeat. Garuda is the same way with WW. If you know the mechanics by heart you can do it with very little strain to the healers. You just have to know your stuff.