That moisture barrier plays a very real role in keeping water, or water vapor, out of the cabin. For all practical purposes, the inside of the door is still "outside". It may be the inside, but remember rain, dirt, water from washing the car and all sorts of other stuff gets inside the door - that's why there are drains in the bottom of the door. That plastic wrap does it's part in keeping the moisture on the outside.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.