I see your logic, and it is working its magic on me. I still would worry about the potential for a mass assault to cause issues, eg target prioritising, I always consider the worst case scenario due to the fact that in the end if it is possible cause a problem most times there is someone wanting to do it (just like cheating always 1 bad apple). I agree that currently the NPC client must see a certain amount of stance changes and even movement, or the rats through to the Ulbernauts would not target and attack any given player let alone follow them. But I would then querry is it to be an active scan for these stance changes, Does the NPC client have a routine to search for this, or is it going to be reactive does someone have to inform the NPC client, "Excuse me guard there is a fight over there>>" which sets off the routines. And again how does the guard differentiate between attacker and defender if someone was being mugged, I know I would not stay in defensive stance if I felt I could over power the attacker and put an end to the fight.
This leaves, however, the messages sent to non-guard NPCs, like rats. Right now, NPCClient likely just drops all fight messages (but already has to check against the list of what to drop), whereas in this case it'd have to filter them by NPC. Still, given that selective dropping by NPC wouldn't require that much processing (if (NPC is not a guard) then drop message), that NPCClient is designed to be independent, that NPCs are still perfectly fine with a response time in the decisecond range,
As for the npc's already just drop the combat stances, I wouldnt know I have not been invovled in the scripting of their AI, I would wonder if they even "see" the combat stance messages potentially this could all be client to client, just like I pick /tell is, I imagine that does not go to the NPCCLIENT unless its /tellnpc. I would envision that if my character was to change stance then it would be issued from my client to the server and then directed from the game server to all those in a given radius of myself. As such it never goes to the NPCclient. My location however would and different NPC's then act as their AI would dictate. I would also go so far as to say taht when the NPCclient is down we still see combat stance changes hence my claim is they are totally independent of the NPCclient.
This is only a guess and definately not an educated one at that so developers feel free to jump in and correct me.
BTW not having looked at any of the coding (My wife will kill me if I start playing with yet another game) feel free to tell me to shut my mouth untill I have looked at it.