In most cases, yes, buying the new edition is just a waste of money. Generally, the main issue with buying old editions is when the prof assigns homework problems directly from the textbook. Very often, the question numbers will change in a new edition and even the numbers given are often changed as well (e.g. they change 15 m/s to 12 m/s), so it can be really difficult to do your homework if you have the wrong edition. That said, in one of my classes, when the prof gave homework sets he would assign them from the current edition, and then also give a list of which questions those corresponded to in the old text and all corrections that needed to be made so that the questions were the same. So some profs are very willing accommodate students with an older edition.
In most cases, you really don't need the newest edition. There usually aren't any drastic changes from one edition to another. Check if there's going to be homework assigned directly from the text, and if not, just be reasonable (i.e. don't get the 2nd edition when the 10th edition is recommended) and you should be fine. If you have the ability to do so, you also probably should make sure that it still covers the same topics. Generally, there's very little change, but that may not always be true.