The problem is that it's not accomplishing that point. Heavy doesn't make it harder to get away from the mobs, especially if you're mounted anyway. I've never stopped to fight a mob just because it cast Heavy on me. You still just run on past, it's simply that you run a lot slower as you go past (then continue to run slower long after the mob forgot about you entirely).
The limited range within which mobs will follow you makes Heavy worthless in a chase situation where the mob hopes to catch up to us. Its usefulness lies in the opposite direction, when a ranged fighter wants to prevent his opponent from approaching into melee range. Neither escaping while chased nor approaching into melee range can be avoided, but either can be delayed a little bit. The thing is that there's no point in simply delaying the former, whereas delaying the later can be quite useful.
So the mobs that are using Heavy are using it badly. That's why it fails to make them any more dangerous and just makes them annoying instead.
If SE wants those mobs to have something that will concern players passing by them, it should be a ranged attack that does some real damage. Or perhaps a skill that knocks us off our mount, as that would be far more likely to result in actually fighting them than a slow effect does.