I feel you're ignoring the advantages of this tool to focus on the speculative scenarios in which it becomes a problem.
I'm referring to issues that actually happened when I used the ability as a head of plot. They aren't speculation.
If the owners don't want more than 2 or 3 PC's engaging in combat against a single NPC at any one time, for whatever reason (sportsmanship, they want the NPC to feel like they aren't getting run over all the time), then they should write the combat rules to say that. I feel like that would be better/more straight forward and less of a mess than what Swarmed By can potentially bring to the table. I don't think it's fair to anyone not just have this clearly defined as a rule and then have people get pissy at players who don't follow this "rule" even though it basically comes down to being a subjective, unwritten rule that somehow falls under the umbrella of "sportsmanship".
I feel you're creating a contradiction with your argument. You expressed concern that the current iteration of Swarm rules will encourage IG behavior of choosing how to engage/loot an enemy, while also stating you do not believe Rules should not dictate in-game behavior.
I wasn't referring to "Swarmed by" specifically with that statement. I'm simply saying that we should not make rules that require people to be kind to each other in-game. That's it. If my character wants to be utterly angry at Tantarus because Tantarus decided to pronounce the word "Green" incorrectly in my character's presence after repeated warnings not to, with the explanation that where my character is from that happens to be the most grave insult one can hurl at another...then my character should be allowed to do that as long as all of the rules for combat and such are followed.
The main point that was brought up is that players should act a certain way so that everyone can have some fun. And while I certainly agree with that, there will be times and circumstance when characters either purposefully black-list another character or characters from loot because THEY CAN'T SAY GREEN CORRECTLY or because they are Hobling and Hoblings are dirty cheats! But there will ALSO be times when the in-game situation dictates that higher level characters should be doing thing one thing, and that is going to, by definition, cut out the lower-level characters through no fault of their own. (Hopefully plot has things they can be doing too, related or not.)
Not all encounters, not all fights, are created equal. Nor should they be.
But right now, a lowbie with a longsword and some gumption can get in on a BBEG and make a dent or two in the BBEG's defenses, or even take them down. Remember when Amory brought down Zakka (that was awesome!)? I don't think that would have happened if there had been a Swarmed By ability in use - and I don't think I need to explain why.
The exact things that I'm talking about happened when the PC's were finally told "this is how you fight this BBEG". You know those players personally - do you really feel like they are the kinds of players who maliciously told people, "Hey, stay back, we need our strongest people only to fight it!" I certainly don't; but the stakes were high enough, and people were into it enough, that that is what ended up happening to one degree or another (and there were some players who were miffed by that). (And, if you recall, there was lots of other stuff going on for people to fight/interact with, including a dragon puppet.)
Yes, sometimes plot can and should create those kinds of situations; it is plot's job to create conflict and strife, but these situations need to be constructed carefully and with lots of thought behind them, because if you don't do that you're going to have a lot of unhappy players on your hands.
I just worry that in chapters that have a historically bad NPC to PC ratio that this will get used in lieu of just adding more body and more per-day skills and abilities.
How long an NPC antagonist lives during a fight is actually quite important, for a couple of reasons.
1) The player of the NPC is going to get fatigued the more times they have to go down, do their death stuff, run back to the spawn point, and come charging back in again as a whole new thing. You have to ration out your NPC's energy!
2) Encounter challenge - the longer an NPC lives, typically the tougher, more challenging the fight is. As plot, you want your PC's to use their abilities, but you don't want to run them out of skills/spells/abilities before that big fight you have planned later in the day.
The primary ways that plot has to increase NPC Time To Live (TTL) are:
More body.
More skills/abilities (defensive and offensive)
Adding rituals (defensive and offensive)
Number of NPCs in a wave/encounter at once
A combination of the above.
Adding in "Swarmed by" to a creature is basically giving it the "Unscaled" tag, because it can literally scale infinitely - which is something that these new rules are kind of trying to get away from.
It's a reason why Golems are so terrible and imbalanced. Immunities, unlimited skills/abilities throw scaling off to the point of being an exercise in futility.