I'm not even a parent, but I saw the book on facebook through a friend-with-kids and was howling the entire way through. I preordered a copy on Amazon immediately after finishing reading the book and I bookmarked it for future reference for gifts to new parents. If it wasn't for the book being on facebook, I never would have heard of it and I probably wouldn't have bought it if I only saw the cover or a page or two on Amazon or at a physical bookstore.
But I like books in printed form, so I usually try out reading a 2-3 chapters of the pirated version, then if I like it I'll buy it.
The book in the article also appears to be a nice gift. You obviously can't give pirated PDFs as gifts :)
You were perhaps aiming for a sound-bite, but saying "It works in very few cases" is inaccurate.