Most people I know in medical school want the newest editions simply so that they "miss out" on any new material. That's valid, but for the most part, pathology hasn't changed much, at least in print, over the last few years.
I'm sure you'd do fine with an older Robbin's. I've noticed that the newer ones just had a little more colorful diagrams and some rearranged chapters. Even if your lecturers referenced only the newer edition's pages, you could probably figure out the corresponding parts.