Personally, I don't know if I've read 36 books in my life. (Although I'm a voracious reader of the internets.)
But some people are trying to support a book-a-week habit. Even if you're dealing with a book-a-month habit, I think the convenience factor might win out. Actually reading what you want vs. reading what you can find and tolerate seems like the Tipping Point to me.
Plus, the fact that you can always be carrying around your "next book", without actually having to carry around your next book sounds like a great option to have.