As I see it, there are only a very few situations in which a firearm would be of avail in a taxicab robbery.
I have known four drivers who have been murdered by BGs. In two of these cases, the robber(s) waited until the cab was stopped in an advantageous location and the driver had put the transmission in Park. Then, they simply shot him from behind without warning, rifled through his pockets, stole whatever they found and fled. One BG had a .38 in a back-pack, cocked and ready to go (so he confessed) so the driver would not hear the clicks made by cocking the weapon. In the other two, the driver resisted the robbers, and lost.
Some time ago I saw photographs from a security camera inside a Fairfax County cab showing a live robbery as it progressed. It was shot in the dark with infra-red illumination, and it made my blood run cold. More than likely if you make a bad call and let BGs into the cab, they will have the drop on you.
Nevertheless, there are times when I do go armed. The killing of one of ours 3 days before Obama was sworn in was one of those times. In those times I carry concealed in a shoulder holster adjusted so I can grab the weapon with my left hand if need be. (In that case, it was evident that at least one of the BGs lived in the immediate vicinity, since the driver was called to a "safe" address which only a local resident would know that a cab would respond to after dark. I was not about to be a sitting duck.)
Veteran cabbies know that the best defense is very high situational awareness and asking the right questions of a passenger and knowing what responses indicate trouble, and how to spot behavior before the fare approaches. One night I was called to a service station, and as I pulled up I could see no one; but then two guys came out from around the side and motioned for me to drive closer. I suspected that it was to get out of range of the security cameras, and I took off. (They didn't call back to complain, either. I think I was right.) Another time a guy walked up to the cab throwing those idiotic "gang signs". I took off pronto.
Another thing about going armed in Northern Virginia is that you likely will have to go to D.C. or onto a military installation, etc. The Hack Inspector's opinion is that going armed violates the City law about drivers being forbidden to carry "dangerous articles", and he has stated that any driver found to be armed will find himself up in front of the taxi board facing revocation.
Situational awareness, after all, is the first line of defense no matter if you are armed or not....