I'm not a collector, so I will respond from a reader's point of view. I don't particularly care for those plastic covers, but if I buy a library book used, I accept it for what it is and know that it will be protected for a long period of time. However, if I come across a better copy of a book, like a really good used trade paperback or a good quality used hardcover that's not a library edition, I will often swap it out for the library one I have.