In University, I remember having professors who pleaded with us to buy the newest edition of the textbook, which was only available on campus in the bookstore.
I've compared editions quite a number of time. Occasionally the differences are stark enough to warrant the purchase of a new edition, sometimes a few changes are tagged on just to churn a profit. The most memorable of this was a textbook for my logic course, where the edition cost more than $50 more than the previous edition (already $150) and had nothing other than two new chapters, which were horrible. Our professor for that course made a powerpoint for fun just to explain how badly the chapters "sucked."
Then there are the professors you can be sure get some kind of commission from the publisher for forcing their students into buying the newest editions, knowing they were garbage.
Anyway, veering into off-topic land now so I'll stop. I hope this rental system really works out and does provide new editions. I spent near $10000 on textbooks in my four years and half of it was totally unnecessary.