Bad-faith question:
Do you actually have the resources required to enforce this consistently, or is this effectively going to become a player-driven panopticon?
I can open the /who list and immediately find an example:
(I don't know the guy, nor have i ever interacted with him, nor am i singling him out)
The problem is that, under the new rule, how is anyone supposed to tell what's actually happening? Is this player helping an alt get a mount? Doing a quest? Pulling the entire instance? Running a friend through? We can't know from a screenshot or from seeing a high-level character in a dungeon. All i know is "he's gaining an unfair advantage over me right this moment"
I wanted to level a crafting alt myself, but now I have to think twice before doing anything close to my level 60 because I don't know how it will be interpreted.
If the answer is "report it and let the staff sort it out," doesn't that just encourage players to report anything that looks suspicious and be angry at each other?