Not all doors (even Main ones) are plug type. The Main Door on CRJs is held in place by camlocks - the edge seal is pressurized. Similar with DC-10 and, if memory serves, 747 cargo doors. Remember Ermonville & Windsor both happened after the cams didn't go overcenter. Check out "Destination Disaster" which should be required reading for all engineers.
And John, you're right, if we ever did depressurize our next action would be abandon aircraft - normally out through the belly via a mod'd avionics bay hatch or the aft baggage bay door.