CnS is used this early to additionally buff aggro with raw damage.

BLM will be casting fire IV at the time you use hard slash, so this CnS raw 450 potency dmg is still valuable hate increase, even if it's not a enmity increased skill. Sacrificing 20% dmg (from Grit) and other 20% (trick, slash debuff) will be returned once you drop grit 7.5 seconds faster than if you had to use one more power slash just for BLM.