I don't know whether you were implying this, but paying for "unlimited" services when you wouldn't otherwise spend the monthly (or annually, etc.) fee on that product isn't necessarily irrational. It may be that a person isn't willing to spend $5 a book on 2 books because a book is only worth, say, $1 to them. But given that returns diminish at a low rate, that person could be willing to buy, say, 20 books at $1 a piece. In that case, paying for an unlimited service that effectively lets them do so makes sense.