aribia

The (amusing in its existence) lobbying group for the hot dog industry agrees (scroll down a bit or just search for ketchup)

http://www.hot-dog.org/culture/hot-dog-etiquette

I think the problem the author is trying to bring up is that if the only place the variable is used is in a recursive call, this should be the same warning as if its not used at all. There's no way it can impact the return value (or side effects) of the function.