Drat! I missed most of these replies, because I didn't get the e-mail alerts that I was expecting. In the webcast, I ended up not talking much about the self-defense angle, because (like many here!) I don't think that law-abiding people should have to justify ownership of a weapon to the government, so long as that weapon can be owned and used safely. So long as a person isn't a threat to others, then his firearms aren't any business of the government (including for extra fees, registrations, and other permissions). I also talked about 3-round burst weapons versus true full-auto. Anyway, many thanks to everyone who replied! I really enjoyed reading the thread.
The video from my webcast where I answer this question is now available on YouTube:
If you like it, please feel free to share it! If you have follow-up comments and/or criticism, I will be checking this thread, so feel free to post them here.
-- Diana Hsieh (Ph.D, Philosophy)