Read this again. Especially the second to the last line.
They already have their guns on you.
I keep coming back to the bit of training video that was circulating around here a month or two ago, where the cop, approaching a bad-guy with gun drawn and *aimed* was bested by the bad-guy with hands out of sight, hiding a gun.
Even from prone, bad guy could move to present gun and fire, besting the cop, essentially 100% of the time. Bad guy has already made up his mind what he's going to do and is only acting. Cop has to *react* to motion, analyze threat, decide he needs to fire. A quarter second is all the time the bad guy needs to get the drop.
I suppose if you're being robbed, it's not by someone with a built-in bias to avoid shooting if at all possible, like the cops. But still, there's something to be said for the element of surprise. Keep 'em guessing as long as you can.
MrB