I don't know about single use phones. I personally wouldn't keep a nonservice phone as my 'emergency' phone unless financially I truly couldn't afford any other option. The reason why is my definition of 'emergency' is a little bit looser than 911s. I want a phone to call AAA (or someone!) because I ran out of gas for example, I would not feel comfortable calling 911, but I would not want to have two small children and no way to call for assistance.
I currently mooch a phone off my brother, who has me on some kind of family plan but doesn't care to charge me the $10 a month it costs to keep my phone on there. Before that though, I had a tracfone and that was the cheapest option I could find at the time, the one year of service deal let me keep a phone for $9/mo I think, it might have been cheaper with less minutes.