If anything, Stuxnet and Duqu have proved that separate (via air-gap) “more secure” networks do not exist. There exists only one network, the Internet, with some parts labeled as classified and with various degrees of slow connectivity to the rest of the World. And yes sometimes the networking device is just a human with a (USB) stick.
This and exceptions that I have to deal with daily drive me closer to a firewall-less world. I am not there yet though.
Managing organized complexity 