Novels about writers: cheap trick? or something of value?

There's been a rush of novels recently that treat as fiction the life (and times) of a real novelist - some of these are good, e.g., Colm Toiban's "The Master," about Henry James, others not so good. Overall, I'm not a huge fan of historical fiction/fictive biography - feels to me like kind of a cheap trick for a novelist. I mean, isn't our mission to create characters and events, not to appropriate lives already lived and invent dialog and incident at will? The technique only works and validates itself if there is what in ed-jargon we call "value-add." What is the novelist bringing to this material to help us understand the consciousness of the writer and the life of his or her times? After all, the writer himself/herself has already opened his/her consciousness to us, through his/her works, right? Toiban added something to James's life (who would read through the Edel bio's anyway?) because James himself was so reticent and personally withholding in his fiction - a whole area of sexuality hinted at but never revealed, much less explored. Then there are pieces like Cunningham's The Hours, which opened up Woolf's life by contrasting her story with two powerful contemporary narrative lines. Last night I started Adam Fouldes's "The Quickening Maze," ostensibly about the life of British poet John Clare - a truly intriguing character because of his madness and because he was one of the very few uneducated, working-class writers of the Romantic era. There have been 2 (I think) recent biographies, both well received. Plenty of material here for a novelist, too, as a biography can't really probe the inner state of the poet's madness, and as it happens this novel is, so far, more about the asylum where Clare was treated/imprisoned and about the director of the asylum, Matthew Allen - a really interesting look at how mental illness was thought of and treated in the 1830s.