Adopting a new textbook edition is like buying a shiny new toy. You're getting the most up-to-date information inside a brand new, unused textbook, so not only is the material ostensibly the most accurate, but it looks good as well. At the same time, however, new editions are expensive, and sometimes not worth the price. So when it comes down to adopting a new edition or sticking with old edition, you have a real decision to make. Here's why you should consider going with the latter:
This is the most obvious benefit, and the most important. The savings your students receive from older editions typically far outweigh the benefits from adopting a new edition. With new editions occassionally not differing very much from their previous editions, students may not even be getting enough of a benefit to justify the raised cost. Since old textbook editions are, almost by definition, used books, they are usually significantly cheaper to the student when compared to a new edition.
By continuing to use the same edition, schools are more likely to have better buyback values. If a school can keep the same edition (excluding packages with one-time use items), that benefits families as they are able to sell their books back for top dollar. Students participating in buyback gives the school access to more used books, allowing the next class to save by purchasing those same textbooks instead of a new edition. Some families with multiple children can even pass those books on to younger siblings.
Even teachers can benefit from from the decision to adopt old editions. With an old edition, instructors can save time and money by not having to create and/or prepare for a new course after they have taught from a specific edition for the first time. They'll even save money on classroom materials, which is a rising cost as more and more publisher are charging for those things.