As librarians, I wonder if we aren't communicating to the user effectively on our collection missions. My small library is not New York Public or the Smithsonian. It isn't even the same as the library ten miles away in a different town. Our library collection mission is supporting popular materials. We also have a strong genealogy collection which does include some preservation. I would imagine that many libraries, even very small ones, attempt some preservation of materials in local history collections. However, that scope probably does not include a career book for women from the 1970s. Does it have significance in a historical context? Probably, but that isn't what my taxpayers have hired me to collect or maintain.
The point of my little tirade is that I am happy to suck it up when civilians don't understand public vs. academic vs. archives missions. I do get frosty when I have to explain it to professional librarians. Understanding the scope of a library's collection is the one of the basic concepts for anyone that manages a collection. For public librarians, I suggest another step. Think of a regular patron that would enjoy the book or find the information useful. If you can't think of anyone then maybe you need to think about weeding!
PS. When was the last time you read or reviewed your library mission or collection statement?