I don't normally read hardcover books with the dust jackets. I don't like the chance that my reading or carrying them around will wrinkle or rip them. With mylar covers, however, there is an extra layer of protection. (Plus most libraries tape or glue then down so they aren't going anywhere.) These covers set library books apart from my personal books. They remind me of all my trips to my neighborhood branch. They remind me of the smell of books. They remind me of seeing kids running into the children's area. They remind me that I have access to tons and tons and tons of books that I would not be able to buy myself.
I love mylar covers for what they represent in my life. Plus, they just sound really nice when you open a book and settle in for a long read.