I don't think the 2nd amendment is just about defense against criminals .... its about defending ourselves from our own government. I don't ever speak of the 2nd as being needed for personal protection from criminals...it dilutes the focus on the primary purpose. Its like hunters complain that the 2nd amendment protects them ... it dilutes the focus.
If it was just about defending against criminals then there would be a good argument that banning certain guns to us (exempting the police and other gov't agencies of course) would, over time, result in only the gov't having these banned guns.