I don't carry one for backpacking but do carry one for dayhikes, especially in the shoulder seasons. It's part of my "just in case I need to spend the night kit"
My basic emergency kit is a 6 oz thermolite emergency bivy, the space blanket, a Golite poncho tarp, and a Montbell thermawrap jacket plus the usual small doodads. I figure that will provide sufficient shelter and protection for an unexpected night out.
I've heard that a really good use for these blankets is to reflect the heat from a fire, which is the primary reason I carry mine, given that I also have the bivy.
Mine is orange on one side so it's good incase somebody needs to find you.