The point of Heavy is to discourage people from just running straight though huge packs of aggressive monsters on their chocobo without a care in the world. I don't disagree with the premise, since it's supposed to make the world feel at least a tiny bit more dangerous and you're supposed to think twice before doing it, but ultimately the game fails at this goal since mobs are too weak and their leashes are too short to make them anything more than an annoyance.

I'd be fine with the Heavy effect if mob leashes were longer and there were some enemies above level 50 roaming the world. Then it'd actually serve its purpose.