Backflow valves aren't the end-all be-all of preventing sewer backup. Like all valves, they should be cleaned/maintained to ensure they will close when required. As well, over pressure if the house is in a low area and from a large rain surge is still possible.
Here's a great slideshow I found on the City of Edmonton page.
http://www.edmonton.ca/for_residents...rValvesPre.pdf
Interestingly, they also have a flood prevention page.
http://www.edmonton.ca/for_residents...n-program.aspx If only Calgary did the same after 2005.