Late reply, sorry. I'd say that in the domestic terrorism situation you describe, even someone completely untrained would still be more deadly taking aimed shots than with full auto or auto bursts. With auto, most of those 30 rounds would impact the ground unless they were shooting into a really, really dense mass of people, and even then you'd have many shots that would be grazing or non lethal hits. And by the time the second magazine went in, people would be clearing out fast.
With a large magazine it would be a different story though, you are right about that. This is why I think that it's good to have fully auto by design weapons be illegal (think the SAW, or the M240), along with magazines above 30 rounds.