I'm not experienced on the subject, but I am a logical person.
Let's say I'm the BG mugging you (so I'm pretty close; as I expect most of these encounters are) and I've got a weapon (maybe it's a gun, but for the sake of my argument, doesn't matter). AS SOON AS you reveal your gun, you make or break my will to victimize you and I will either surrender or attack immediately.
So, if you carry with a round in the chamber, you're in the fight right away.
If you don't, it is your responsibility to create/maintain distance between us until your weapon is ready (i.e. chambered, cocked and aimed).
BUT, if I, the BG, am just looking for an easy victim (as I likely am), I'll probably back down after your gun comes out (especially if I don't have one, which not all BG's will) IF you give me an opportunity.
Remember that easy victims make a criminal's job much easier, and he knows there are plenty of other people put there who aren't carrying like you are, so there IS a possibility he would flee if you pulled out even an empty gun.
Of course this debate will continue and there is sound logic to each choice... except NOT carrying