The good thing about having a real copy of the book is that it may contain notes in the margins. (There were rumours of an update to this book but I haven't heard of it being released.) Elders will usually have their copy with them when they meet with the CO or at KM Training sessions. It's at those meetings that the WTS will have some additional tid-bits for elders to note directly in the margins of their book. These are the secret instructions that come through letters to elders via the CO. To have a book with these additional notes is very useful.
I wish I'd have kept mine. I was still a loyal dub when I resigned and handed in my book with my letter of resignation. The on-line versions are a good source but they lack the 'special notes.'
3rd