I'm with JJ all the way on this. My only addition to the conversation has to do with my companions.
If I'm the only one armed, then I get myself and my companions to a side exit. Anyone who tries to shoot us on the way will get well-aimed return fire until my companions are out. If I'm still alive, I get myself out.
If one of my companions is armed, then the above still holds true, although we'll leapfrog to the exit.
I happen to have the arguably beneficial advantage that myself and every one of my friends who carries was trained as an infantryman by either the Army or Marine Corps. We all know the basics of engagement; to shoot, move, and communicate. I can only hope that these modest skills save us and our loved ones.
I can't imagine myself sticking around to slug it out with the bad guys without a rifle of my own. As has been mentioned, if enough of these incidents occur, we may find ourselves walking around with rifles slung over our shoulders, but even then everything depends on a critical assessment of the situation.