From what we're told from Obi-Wan, and what we can infer from everything else we see, we get all the history on the character of Anakin/Vader that we really need in the OT. Or all that I need, anyway. Sure it was awesome going back to see how things unfolded (or at least the prospect of doing so was awesome), but still, it didn't add much. Even going by just the OT he was a complex character: an admirable Jedi (undermined by what we see in the PT), best star pilot in the galaxy, a cunning warrior, that gave in to temptation from the dark side, though the good still lingered under the surface. By ROTJ he's stuggling with what side to choose, and only when push comes to shove does he cast aside Vader. Not many villains can boast that sort of complexity.