It was an exploit, regardless of whether or not people left. It was not "tagging" mobs, as anything inside an instance is automatically tagged by your entire party. Imagine the nightmare of having people leave the party and then reinviting them before they were hearthed, only to have them leave -- and so on. O.O
If that is how they worked it, then that's just crazy. And it doesn't change the fact that it is taking advantage of a situation.
Tagging mobs in the world is a whole different story.
/stretch Weeeell, time to clock out and go home to see this thing they call Wrath of the Lich King. XDXD
Gaiwyn