Idling is appropriate in one case: when you're idling on your own preformed fireteam with prior consent of your team mates. Sometimes a matchmade player can do more damage to strategy than an idle friend. Plus the idle friend has a way of charging in at the last minute to grenade the boss for the kill. Those of us who are in the fight then get to race to beat the boss before they arrive. And I'm not clearly one or the other. I don't usually always run, but I usually only kill enemies that can't be avoided, and I've also turned into a 100% runner for summoning pits.