Cats and to perhaps a lesser extent rogues and even shamans have the potential to pick and choose when an AOE attack would be appropriate over single target attacks. This is because their AOE attacks are of a hit once, damage everything once variety.
While your mages, warlocks and probably some hunters are channeling an AOE mobs might move or die leaving AOE inefficient due to fewer targets. We felines and those with similar mechanics have the choice of not hitting our AOE ability unless it will actually be worth it. Thus at the end of the day we will usually come out ahead even if the channeled AOE types were putting out a higher peak damage.
Sure trash DPS isn't very important, but putting the extra thought into when to do what certainly makes it more fun and interesting and hitting a button and waiting. The higher performance is just a beneficial side effect. :)