"I think a 357 in 4-6" and a 12g is adequate for anything."
Unfortunately, that just isn't the case. These rare instances are the reason virtually every (at least that I've seen) police officer carries a patrol carbine in their car.
The average citizen doesn't always have the luxury of waiting for the police to show up and save them. Just the same, the patrol officer doesn't always have the luxury of waiting around for SWAT to show up with more fire power.