Keeping everyone on the same edition actually serves a purpose though. In one class (physics I think) the teacher foolishly allowed people to buy an old edition of the book rather than the more expensive newer edition. And then every class ended up being "OK, look on page 141. Errrrrrr... page 175 in the old edition. Do problems 1-10. Ummmm... problems 2-12 in the new edition, skip number 7."