For PLD, a turtle set would be Max DT, DT-II, Magic Evasion, Physical Defense (this matters a great deal but probably not very much to PLD), Block Rate, Parry Rate, Magical Defense… Shadow Cape?… Basically going outside of the norm that allows for a chance at 0-Damage.
It’s basically an EMO Button (Emergency Manual Override) or the classic “OH ***” set that throws as much of a wall up as possible to survive a boss’s tantrums or “*** You” attacks and generally not so great for enmity.
I think it’s useful to have one and you can get one ready for specific fights if you want to. X boss does “Fire-Blast of Stupidity” at exactly 22% HP? Turtle set with *** or MEVA,MDB and Absorb Fire. Stuff like that.