They updated the in-game tool tip to say that the Book of Cain is there for you to identify multiple legendaries with. However, in play, I rarely ever find multiple legendaries at once, and usually I enjoy 'unwrapping' each legendary because the new legendaries feel quite special, and there are a lot of new ones (and the old ones have new and improved abilities). As a result, currently there's not much point to the Book of Cain. Still kind of cool to have though. It probably won't go away, because usually stuff doesn't get deleted from game code unless the devs are specifically getting rid of something. The Book of Cain isn't really useful, but it doesn't harm anything, so I doubt they'll get rid of it from the game.