Given that plastic is a much softer material than metal, it's not like leaving it on is going to protect the metal hardware significantly. In other words, anything hard enough to scratch the metal hardware would likely scratch it regardless of whether a thin plastic film were on it.
Also, isn't the point of a Birkin to convey status and wealth ("I'm wealthy enough to spend $10000 on a handbag"). If that is the case, the person should the wealthy enough to treat the Birkin like a regular handbag and get dings and scratches on it ("I'm wealthy enough that I don't care if I get scratches on this $10000 bag, I can just buy another one"). If one buys a Birkin and then babies it excessively for fear of hurting the resale value, that doesn't go with the image of wealth that Birkin is supposed to convey.