Two of my favorite passages from Bruce Schneier’s Secrets and Lies: Digital Security in a Networked World:
Several years ago Microsoft made a big deal about Windows NT getting a C2 security rating. They were much less forthcoming with the fact that this rating only applied if the computer was not attached to a network and had no network card, had its floppy drive epoxied shut, and was running on a Compaq 386.
Large gaping security holes are okay if the probability of attack is zero. (Tokyo is still vulnerable to attacks by giant fire-breathing lizards, for example.)