Happy 2013, I hope.
The answer is dead simple: We need 30-round magazines because our oppressors have them.
The Founders lived in a country that was freed from British tyranny by citizens with guns who organized into militias to fight it, and they envisioned such militias would always exist to do exactly the same thing--fight back against tyranny. That is why they enshrined the right to keep and bear arms in the Constitution.
When you break it down succinctly, these 18th century citizen militias were able to achieve victory because of four things:
1) They had a thirst for freedom. It would have been easier to simply let themselves be oppressed and fed by the Crown, and a lot of Colonists wanted to do just that. But rolling over and begging from government was not what America was about.
2) They had effective, layered leadership. George Washington was a savvy tactician and brilliant strategist, but his true greatness lay in his ability to motivate and energize his subordinate leaders and their forces.
3) They enjoyed much shorter lines of communication and supply than their oppressors. Ultimately, the British crown decided that given the economic hardships and logistical complexities of an overseas war and the tremendous expense of maintaining their vast empire, it was no longer feasible to fight the colonials. Had the king made continuing to hold the American colonies his highest priority, things would have turned out very differently.
4) They had the same weapons technology their oppressors had. The British army was oppressing colonial citizens with bladed weapons, muzzle-loading cannon, and single shot muzzle-loading muskets with bayonets. They Colonials fought back with the same.
The Second Amendment is not about personal or home defense, shooting sports, hunting, or collecting. It's about protecting freedom from those who would seek to diminish it. It's about fighting tyranny and oppression. That brings us back to where we started: We need 30-round magazines because our oppressors have them. We've already let our oppressors rob us of automatic weapons, supposedly for our own good, and now we would surrender still more to them?
This battle is not about guns per se. It's about the choice we face: allow government to control every aspect of our lives under the false promise of added security, or take charge of our own lives and of the government that seeks to rob us of liberty.