Re: seat recovering question
I agree with the anti-abrasion, ease to slip-on, and tight fit theories. IMO, the "waterproofing the foam" thing is bogus, because the plastic is going to eventually degrade and/or split negating any "waterproofing" effect. And in the meantime all the plastic is going to do is trap and contain moisture in your foam.
IMO you are better off spray-gluing a layer of dacron batting to the foam. Performs all the functions of the plastic, fills out the vinyl better, and doesn't trap water like a plastic bag.
EDIT: and for when people reply with "but manufacturers use plastic instead of dacron," well yes, it's cheaper, and it only has to last until the warranty expires!