The interesting thing about security is that you don't want it until something bad happens. It is like insurance, until you have been caught without it, you don't understand what you have being paying for. Security is the same thing, and VoIP security is double so. Most people we talk with say, "I don't care if someone can listen to my conversation."
The amazing thing to remember is that not only can they listen to it, they can record it and then piece it together to "make" you say something else. While not a big threat at first blush, imagine that we have a "fast computer" in a few years, and someone has been slowly recording other people's conversations. This fast computer could pretty easily handle the computing cycles needed to be able to simply "speak" words typed in your "voice". Can you image what it would/could say? Now imagine that it gets into the hands of a crime syndicate that wants to do nefarious things with it. Anyone thinking about using VoIP needs to think about securing it.