As others have said, the purpose of the door heaters on the NG is solely for supplemental heating.
The heaters on the pax/service doors are inline heaters while the overwing exits use electric blankets. They are not operated by the cabin/utility switch. They are energized when the plane is in the air mode and one of the pack flow control and shutoff valves is open.
I'm pretty sure its a standard feature on all NG's.