I just find the whole debate about books like this interesting because it raises questions about what is essential to the story and how much we're willing to allow people to take license with it.
For example, the Lincoln play that Selby did last year had the President being visited by ghosts, spirits, visions, etc. and conversing with them. That is arguably an effective device for demonstrating how he struggled with competing ideologies about slavery and the war, but, while not as extreme as what this latest novel does (as I understand it, haven't read it), it's still superimposing a paranormal/supernatural element over historical events.
As for the manipulations of literature, that's not terribly different from what fan fiction does or from what Gregory Maguire has turned into a cottage industry, but we still have our debates about fanfic that goes too far, takes too many liberties, etc.
