There is an exception to what the author is saying: make the cost of adding data for the user exponential (or at least non-linear). This is what Crashplan does with their unlimited backup model. It's unlimited, but at some point (number and size of files to keep track of), your computer will be limited by its processing capabilities in a manner that you can only solve by throwing more metal at the problem, which becomes uneconomical for the user. (Source: the internet, I can't remember where, but might find it when I'm back on my computer)