Well, you can't have a library without expecting people to return the items they borrowed! Our time-honored date due slips are ungummed. Traditionally, a small line of glue across the top of the slip holds it in the book. The librarian, of course, expected to lend the book many more times than possible with a single slip - so this method of attachment made it easier to replace the used slip!
Of course, there are most probably a million new innovative ways to use these slips - particularly for baby showers or baby gifts.