Ah. From the way IGN said it, I would have thought they would actively go around killing instead of just settliong down somewhere, lol.
I haven't personally seen it happen (meaning that it probably takes a number of hours), but I would imagine that the mobs despawn after a period of time. Otherwise you could literally have an entire zone overrun 100% of the time, which would stop players from leveling completely until some insane number of mobs are killed and the quest giving NPCs respawn... only to have the entire thing happen again that night.
One of the best parts about this game is the soul / role system. You have a number (8, 9?) of souls (think class or talent tree) to choose from for any of the four classes, and you can combine any three of those at your leisure. Each of the four classes have souls that are good for tanking, and all have either a healing / support soul, or a tanking soul, or both. You can purchase additional roles, allowing you to carry around up to four "specs" at any given time. So, my rogue for example, is an Assassin / Nightblade / Bladedancer on his main role (melee DPS / stealth), and a Bard / Nightblade / Bladedancer for his secondary role (healing / support). On my third role, I'll probably take Riftstalker / Bladedancer / something else for tanking. For a price, you can even respec a role, so there's absolutely nothing stopping you from trying combinations to see what play style you like best.
The beauty of this system, other than giving you a LOT more character customization than other games, is the fact that if you role a Rogue or Mage, you're not strictly DPS for your entire career. If you're a Warrior or Cleric, you're not always expected to tank or heal (and you can DPS just fine). You'll still see some groups that just cannot find that tank or healer that they need for their group, but I find group composition problems to be a LOT less of a bother in this game than WoW or similar games.