Clarence - 6:49 am on Dec 8, 2012 (gmt 0)
Google: "These networks compete for your ad inventory, potentially increasing your total AdSense revenue." Take note of that word "potentially". In reality they discount you traffic. It's better to give more relvant advertise that space for cheaper "who will get clicks" then to stuff it full of super direct cheap CPM advertisers.
I've seen Ad Network's bidding $0.03 CPM getting impressions on my site. I would personally prefer to give Free Ad Space to Charity then to accept such low earnings.