Even if you don’t see major changes that are obvious from the table of contents—addition of new chapters, updating of recent history, reorganization of material—there may be small revisions throughout the book. I have been paid to edit a textbook’s second edition that not only incorporated recent studies but also contained a quantity of refinements, correction of small errors, and updating of certain terminology throughout. It also had a new index.
By the same token, my film studies teacher (about 2 years ago) told us that if we had the 11th edition, we were fine, and not to bother with the 12th unless we especially wanted the new one. He gave assignments by chapter and not by page number, so there was no problem following the syllabus. The difference there was mostly in references to and still photos from recent films. I wouldn’t have taken the chance if it had been in a class with volatile subject matter, such as anything scientific, technical, historical, or political.