I still say the plastic wrap is a manufacturer's cheapo shortcut for quick assembly. At best it will deteriorate quickly and do nothing for you (beyond the initial quick slip-fit). At worst it may trap moisture inside a cushion.
IMO, best cushion design is to use 1/2" or so thick polyester sheet batting wrapped over (and spray-adhered to) your foam on all seating surfaces instead of any plastic wrap. This serves the same slip-fit function as the plastic wrap AND it will actually hold up to prevent the vinyl-to-foam chafing over time described by a previous poster. Furthermore, it will make the vinyl "fill out" and look better (like a well-upholstered couch), AND it will let moisture through, which is what you want, because you want the bottom (or bottom edge on a vertical cushion) to be constructed with a water-releasing mesh something like that posted by Bruceb58 above.
This will cost more, but you are not a high-volume manufacturer looking to save every labor and materials penny and sell something that will wear out in three years so the customer has to buy another one.
All just my opinions, based on research and personal experience dissecting crappy manufacturer boat seats and cushions and re-doing them far, far better.
