Don't forget geography books (and for that matter any history book which may have a modern map somewhere). We were still using books with the USSR on a "present-day" map as late as 2000 in our school. (The book, however, was medieval history so the content inside of it worked fine).
There are other reasons schools may adopt new textbooks. New mathematics books include technology exercises for newer equipment--even if the older books usually do offer more practice problems and sometimes explain things a bit better (I was always a fan of the Mary Dolciani math books, even if they couldn't be used with a TI graphing calculator). There are also plenty of books nowadays that are better suited to NCLB standards than their predecessors. Several even include test-prep exercises within the books (I am currently tutoring a student from a mathematics book which not only includes said exercises but is also a "Pennsylvania edition").
Schools will often donate textbooks to less fortunate districts/schools. We did this with a set of social studies books in 1997 and again with hundreds of secondary math books in 2005. However, if one book gets left out and you ask a teacher about it, they just might give it to you. I have a Prentice-Hall geometry text (from 1998) because this situation came up--much like it does when we get surplus computers.