I'm not sure what you're arguing against. As I said, it is explicitly stated that there's no definition of the number or pattern that constitutes "persistent", which quite literally means it's at the ref's discretion. Another useful source I've found is a question and answer on exactly this situation on Asktheref.com.
http://asktheref.com/Soccer Rules/Question/24316/
This is a named and licensed referee saying exactly the same thing as what I and a couple of others have tried to argue in this thread. You can continue to dismiss this on the basis that it's not literally stated, word for word, that a ref can book a player when he had warned a team for fouls by other players, or you can look at all of this information from a distance and come to a sensible conclusion. The choice is yours.