My understanding is it's common for sites to use a bunch of ad providers, because not all of them will offer up good paying ads on a given page. So you try for the most profitable ad provider, and then another ad provider, and then another one, and then maybe if none of them pan out, you show the Google Ad, which gives you something to fill your ad box.

Source: I know a guy who runs a blog who wrote Google+ posts about doing it.