The more I think about it, the more I think that this is what it ultimately boils down to: Lucas put Hayden into ROTJ purely as a link to the PT, and only started to rationalise it later. It's no coincidence that the so-called "Original" Trilogy DVDs hit the market in the run-up to the cinema release of ROTS. Lucas hoped that seeing Hayden in ROTJ would whet our appetites for ROTS. The logic of this change (or lack of it) never entered George's mind. Once again, he's relying on the fans to do his work for him.