I hate how prequels feel they have to explain everything that takes place in the earlier published but chronologically later films. It's like the only events of significance in the lives of the characters take place in very brief windows of time. Explanations of Professor X's paralysis come to mind as bothersome and inconsistent with previously established movie canon.

Wonder where I'm going with this? My point: why was it necessary to show Palpatine' facial damage in Ep III? I don't think an explanation has ever been wanted or needed. The EU explanation (from Dark Empire-ugh-but logical) was that his massive usage of the dark side had consumed his body. Though I was young when I'd first heard this reasoning, it seemed sound to me, and I feel it would have been my ultimate assumption had there never been any EU. I wish that someone could edit out the Emperor's facial damage throughout the whole movie. I would feel completely content if I'd watched the films in chronological order, left off in ROTS with Palpy looking relatively healthy, then picked up in ESB and ROTJ looking decrepit. That would certainly be a challenge for any editor to fix, though.