This is a reasonable point of view, until you notice Linux desktops evironments don't provide applications with a method to detect if they are running on a private network or not (See Windows Home, Office, Internet network settings). Then a non technical user start Rythmbox, enable music sharing, and it works perfectly on their home network but then decides to buy a WAN card/USB stick and suddenly all the music is being shared to the world. I wish NetworkManager could do something about these situations, maybe the default should be the public zone for interfaces that receive public IP addresses.