Writing a book in someone else’s voice allows the ghostwriter to abdicate responsibility for anything that is said. The release from that responsibility compensates for the inability to express your own views. In one way it makes it easier to tell a story dramatically and to introduce readers to the personality of the subject, but it is also an act of cowardice, a way of hiding behind a mask. It makes it much easier to express outrageous opinions, to justify shocking behaviour, if you are using someone else’s voice and letting them face any hostile responses that might come from readers.
Maybe it is the same with fiction. Some stories would be hard to tell effectively without a narrator. Would Vladimir Nabokov have been able to make Lolita palatable if he had viewed Humbert, Lolita’s seducer, objectively? By allowing Humbert to be the teller of the story he could make it easier for the reader to understand why the man acted as he did, perhaps even to empathise with him despite the fact that his crime would be despicable to virtually everyone. The glamour and drama of life at Brideshead and Jay Gatsby’s mansion were all the more evocative for having been viewed through the eyes of the impressionable young narrators, Charles Ryder and Nick Carraway, who were actually there and actually affected by the events that occurred.
By taking on the role of a ghost, the writer is effectively, and perhaps cravenly, handing over responsibility for the truth to the narrator or client.