It's far from useless, but from a BLM and WAR perspective, the two jobs I've played with MCN, your 100 potency damage can cause me to lose:
- Hitting that mob with a 100 potency Fire II, and a 1.9x AF3 damage modifier
- Hitting every mob except that one with that same Fire II, because it was my target and it's now out of the group
- Hitting every mob with that same Fire II, because you knocked my target behind me and spell was interrupted
- Hitting that mob with a 90 potency Overpower (120 -25% from defiance), and an autoattack if it was my target
- Hitting that mob with a 200 potency Steel Cyclone
It will often take away more damage from a party than it will add if there's a knockback that isn't a finishing blow.