It takes a good chunk of resistance to start to see an impact and even then, I don't know if it's a flat out reduction or percentage based.

One time you may consider using it is in Ifrit Extreme if you get chained to a tank and they're about to swap. Since you might reposition yourself to be closer to this tank, you might use Apocatastasis on the tank to reduce the damage of Incinerate (or triple Incinerate depending on timing *unenthused yay*).

But yeah... that's too many ifs lined up to use up an ability slot.