It's called an airgap, and is easily the most effective countermeasure to cyber attacks, and, in conjunction with physical security and careful 2-man control/people management, should certainly be relied on.
I'm positive you can fit an interesting virus on a double sided one (the most common I believe).
"Your PC is now Stoned!" http://en.wikipedia.org/wiki/Stoned_(computer_virus)
So yes, a network designed with the assumption that it will be constantly barraged with attacks is probably more secure than one designed be people who (subconsciously or not) discount the possibility of malicious traffic ever occurring.
The reality, is that you want to do both - have a rock solid firewall, and secure your internal servers.
In the case of a Nuclear Missile Silo, I'd like to believe that as much as possible was run with switches, dials, and manual controls which require physically penetrating the perimeter of the silo, and having all sorts of advanced credentials, procedures, and codes to actually accomplish anything. (Said PAL codes being supplied by the President or their designate).