I am of mixed opinions on this strategy, especially from librarians. It often ends up being more work for the instructor to try to reconcile different editions for students. Some editions have significant changes, leaving students with older versions at a disadvantage and the instructor has no way of knowing that. Other updates are nice but not crucial for the students. Students need to check with instructors before sticking with an older edition. Sometimes instructors prefer an older edition but the campus bookstore will push to "upgrade" to the newest edition.