In addition, cities with recycling programs can actually sell the material they gather back to companies, literally turning garbage into money and creating a revenue source for the city[2,3]. One important component in this is that the cities can get a higher price for higher quality recycling streams, where the output is all composed of one type of material. This is why, even though St Paul is now single-sort, I still separate my paper from the other recycling. It helps keep the paper higher quality by avoiding getting it dirty and wet, and costs me no real effort.
[1] http://environment.about.com/od/recycling/a/biodegradable.ht...
[2] http://arstechnica.com/science/2015/03/recycling-in-the-us-a...
[3] http://arstechnica.co.uk/science/2015/12/recycling-matching-...