My view would be if they are books that you will read a lot in the future (and don't have in ebook format) and / or would be ones that you would recommend to people and want to lend out then it makes sense to keep them on hand - if you won't then they are more just taking up room for the sake of having them when someone else could be loving them even more. I love books, I love the feel of holding a book as I read it - but I read them for the words, either for the story or the information. If I can pick up the story without picking up the book it makes me just as happy.