For me, books are for reading - not to act as decor or show off my reading habits to visitors.
I read about 70-100 books a year, so I couldn't possibly keep hard copies of them all unless I moved to a castle. I probably read about 75% of my books on a Kindle and about 25% are paperbacks, which I box up and donate as soon I've read them.
I love reading - my degree's in English Literature and I have a book reviews blog, so clearly reading is incredibly important to me. But for me, the important thing about a book is the words in it, not what it looks like in my house.
In our old house, we had bookshelves covering two full walls of our living room and in one room upstairs. Then we both got Kindles. We've since either sold or donated several thousand books.
Now, we have one wall of shelves which are in the spare room I use as and office, but they're for reference books, coffee table books, special editions, art books, niche or sentimental books no longer in print etc - the kind of thing you can't read on a Kindle, basically.