Easy answer: to up the difficulty on low-level quests without having to run it multiple times. If I can get someone to open the Korthos island quests on elite, I am only running those darn things once to be sure.
Again, if you can't keep yourself alive while zerging, you shouldn't do it. Even when I am not zerging, I have no issue with people who do so as long as they follow that rule. I honestly don't see a valid argument against it if that rule is adhered to.
Those that complain about zerging are being as selfish as those that zerg. It's still a matter of trying to control the pace of the group.