Well, that's what they do -- except that in harder areas, they have more health. The reason being that in a higher-level area, an exploitable tactic would be just to go in, avoid the enemies, and carpet bomb the spawners so that you don't get any more enemies. It's the sort of thing that I imagine really expert players trying to play at +10 levels over their civ level would do. That's been my reasoning, anyhow.