It is true there are cases where pulling out a gun will deescalate a distuation. If i was on the jury pool and the guy pulling the gun was in danger, then I'd vote to let him off the hook. But legally speaking, brandishing is not an option.
As far as when you can draw and fire, thats wholly up to you and your judgement. A guy with a bat? Probably 30 feet. A guy clenching his fists? Probably 30 feet. A guy with a handgun? Probably 75 feet. Why 75? Its the farthest I know I can shoot and hit center mass. Which brings me to my next point.
Practice. Practice as much as you can afford to spend, both in time and money. I shoot 400-500 rounds a week (I'm a nutty rightwing christian who wants cold dead hands, but not white), and I've taken qualification tests at our range to prove to them that I know how to handle a gun safely and effectively. One test includes drawing from a holster, firing 2 shots in seconds at 75 feet and hit an 8in diameter circle. That is how I know I can comfortably hit center mass at 75 feet. Beyond that, I don't know. Why 30 feet? I know I can comfortably draw and fire into that 8in circle with a double-tap within 3 seconds.
No one wants to be caught unprepared. If I pullover because someone is being dangerous, and they pull over, it would not be tactically advantageous of me to sit and wait in the car. Step out of the car, try and get into a highly visible location, call the police, put hand on grip and be ready to draw while shouting "Stop! Drop your weapon! Do not approach or I'll shoot!" Here's a secret about calling the police: If you can call them and talk to dispatch about where you are, great. If not, then you are probably already shouting at the attacker. Dispatch hears this, they record, and if you have the disparity of force on your side and stop the threat, that recording may be useful later.
Back to the main topic of panic gun buying. I wouldn't be too worried about shotguns, handguns, and bolt action rifles. Be worried about assualt rifles and submachine guns. Be worried about .50cal's, full sized machine guns. No one absolutely needs them, but we have a right as American citizens to be armed to the teeth. I believe the founding fathers knew that we may have to face oppression once again, and wrote, not discussed, not put into the editorials, but wrote in our Bill of Rights (yes, rights, not provisions by the government for the people, nor provisions by the people for the people, but absolute, no matter what, rights), that ever important 2A. Whether its oppression by other people (criminals), opression by corrupt governments, or even opression by our enemies, we do have a means of equalizing the force.