IB will actually be extremely effective if used well. I'm thinking in Titan HM, which is far from "endgame" but relates to the vast majority of players, you could use IB for... every Mountain Buster? While a PLD may have more cool downs they certainly don't have enough to pop one on EVERY mountain buster meanwhile a warrior if using their brain will have IB up every time.
I would imagine the ideal time to pop IB would be after the first auto attack used prior to Mountain buster. If after landslide Titan doesn't AA and you feel like MB is coming... just pop it then.
My point being a 20% DR skill that is nearly spammable is going to be extremely effective when used well.
