The doors of police cruisers sometimes do offer cover and not merely concealment.
Beginning a few years ago Ford started offering the option of factory installed Kevlar panels in the doors of Crown Victoria Police Interceptors. I believe this was an attempt to capture some of the market discovered for such products by aftermarket installers.
They were somewhat expensive, starting out at over $1,000 per door, with the option of ordering panels in both front doors or just the driver's door.
The manufacturer brags about its products here:
http://www.body-armor.com/vehicle.php