So what is PLD good at?

[Paikis]

I think it's more of a hunch than real effectiveness. The purely hypothetical situation is enough to make healers uncomfortable and seeing bigger damage on the tank is scarier for them.

Since when does maths care about people's feelings? The difference in mitigation/healing received based on Defiance v Shield Oath is so small as to be effectively meaningless, regardless of how uncomfortable it might make people feel to see bigger numbers.