You do need it.
It isn't only a moisture barrier. it also, if properly installed, directs water back into the door to the drains rather than running down the inside of your waterproof ABS panel and into the bottom of the upholstery and down to the carpet.
Even modern cars with ABS door cards still have the plastic sheeting.
Sic friatur crustulum
'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.