Interesting set of thoughts here.
I have an interesting perspective on this as I *have* been attacked and pummeled in an attempt to rob me, while armed (I at the time didn't have a CCW, so was carrying a knife, Spyderco Chinnook to be specific). The attacker attacked me from behind (by wacking me over the head), and tried to wrestle my briefcase (secured across my body with a strap) from me, failing in that decided to 'rough me up' a bit.
I decided, given the situation, not to draw the blade, as I felt at the time my only option, given the poor tactical situation (me already down on the ground) would have been to try for a killing blow which was low percentage, or run a significant risk of having it taken away from me. I opted to protect my vitals, and, as we were in a fairly public place, hope it didn't last long.
This experience ended for me in a concussion, along with a 13 stitch gash in my scalp (from the initial attack, didn't see it, it was from behind, he used a board which he then dropped), and some bruised ribs, from being kicked etc.. once I was already down. By the time I was on my back, it was impossible for him to wrestle my briefcase away from me, as I fell on the strap, so I didn't even lose anything, except some blood and dignity.
This has been a huge motivating factor for me to take some self-defense courses, carry pepper spray at all times, and get a CCW.
However, in the situation, I still think it was right not to draw the knife. According to the police officer who interviewed me after if I had stabbed him, given he was 'unarmed', I would likely be looking at assault/murder myself. Any situation he could see that would have given me an opening tactically to use the knife, would have also given me the ability to disengage. I was actually complimented by the officer for my restraint (thanks sir! how bout a ride to the hospital?) I think that shooting an unarmed assailant would probably be looked on in a worse light.
This significantly underlined, to say the least, the drawbacks of a knife as a self-defense weapon. I strongly feel now that there are two classes of situation, those that pepper spray will handle (unarmed assailant), and those that I'd really like a gun on me, thank you (armed assailant).
Now that I have a CCW, I'd be very leery drawing on anyone unarmed, working on the reality this has pounded into my head, that you can't, even if you have it, draw a weapon unless you are completely prepared to use it, and to defend that decision and all of its consequenses.
I second and third those who recomend pepper spray in the situation, as well as yelling, or even wacking the guy over the head with something like a chair if you really feel the other person's life is in danger. I just think it is a bad time to go OK Corrall.
Oh, and also, trust me, it is a really good thing to watch your six! Learned that the hard way......
Just me $.02, probably worth even less. Interesting discussion, though.