I can see buying just the prequel trilogy....
Several people have stated intentions to re-do (or do in the first place) prequel fan edits using the Blu footage. Theoretically you should own the Blu if you want to download those future edits, and if you have no use for the current OT on Blu (say, you only want to watch dark_jedi's GOUT sourced OT), there wouldn't be any reason to buy all six.
Unless you want the super secret bonus 9th disc, of course.