It doesn't make sense that The Force would render Anakin's ghost looking like Sebastian Shaw, as he NEVER could have looked like that. At the end of Revenge of the Sith, Anakin is burned almost beyond recognition. He is missing three of four limbs, and has all of his hair burned off. When Luke removes Darth's helmet at the end of Jedi, he's hairless and pretty creepy looking, yet the Sebastian Shaw ghost has hair and looks pretty normal. Is this supposed to be some kind of Force Fantasy of what he WOULD have looked like?
It makes more sense that The Force would restore Anakin to the way he was before he was consumed by the Dark Side. But bitter fans who hate the prequels will find any reason to take another shot at Lucas and his choices (some of which, I'll admit, run the gamut from inspired to misguided to downright awful).
By replacing Shaw with Christiansen, it gives objective or newer viewers something to connect the trilogies together, and give that ending greater emotional weight.
I don't hear anyone complaining that the terrible-looking Emperor from Empire Strikes Back was replaced by Ian McDiarmid (the guy who played the Palpatine in 4 other films) for the DVD release. A great idea that ultimately affects the quality of the film more than the timing of Han Solo's gunshot.