@Seraphus : Maybe I haven't played enough MMOs, or maybe not the right ones... but for the ones I have played, aggroing every monster on the map would just result in being ganked in short order. That said... a few circumstances come to mind...
The Tir Na Nog graveyard zombies in Mabinogi were a fun one. Slow-moving, so I could run around them in circles to bunch them tighter together then Windmill them repeatedly to kill them... though getting more than 5 at a time was tough. For most other monsters in Mabinogi, trying to aggro several simultaneously was downright suicidal.
Guild Wars was a bit more forgiving, though it was still easy to get overwhelmed if any of the monsters could strip enchants. Still, now I think about it, both Mabinogi and Guild Wars gave me the option of giving monsters the
*teleports behind you* Nothing personnel kid treatment. No need to aggro large numbers of monsters when one can just teleport from monster to monster backstabbing them.