Full disclosure is certainly not perfect. By definition, it increases risk in the short term, which is why opponents are able to make persuasive arguments against it. Like all security, it is a matter of tradeoffs. Does the long term gain justify the short term risk? As I'm fond of saying, human beings don't so much solve problems as they trade one set of disadvantages for another (with the hope that the new set isn't quite as bad as the old). There is no solution here, only a less disadvantaged system.
Now I'd like to broaden the subject even further, and apply the concept of open security to national security. With respect to national security, the stakes are higher and thus the argument will be more difficult to sustain. If people are unwilling to deal with a few computer viruses in the short term in order to increase long term security, imagine how unwilling they'll be to risk a terrorist attack, even if that risk ultimately closes a few security holes. This may be prudent, and it is quite possible that a secrecy approach is more necessary at the national security level. Secrecy is certainly a key component of intelligence and other similar aspects of national security, so open security techniques would definitely not be a good idea in those areas.
However, there are certain vulnerabilities in processes and systems we use that could perhaps benefit from open security. John Robb has been doing some excellent work describing how terrorists (or global guerillas, as he calls them) can organize a more effective campaign in Iraq. He postulates a Bazaar of violence, which takes its lessons from the open source programming community (using Eric Raymond's essay The Cathedral and the Bazaar as a starting point):
The decentralized, and seemingly chaotic guerrilla war in Iraq demonstrates a pattern that will likely serve as a model for next generation terrorists. This pattern shows a level of learning, activity, and success similar to what we see in the open source software community. I call this pattern the bazaar. The bazaar solves the problem: how do small, potentially antagonistic networks combine to conduct war?Not only does the bazaar solve the problem, it appears able to scale to disrupt larger, more stable targets. The bazaar essentially represents the evolution of terrorism as a technique into something more effective: a highly decentralized strategy that is nevertheless able to learn and innovate. Unlike traditional terrorism, it seeks to leverage gains from sabotaging infrastructure and disrupting markets. By focusing on such targets, the bazaar does not experience diminishing returns in the same way that traditional terrorism does. Once established, it creats a dynamic that is very difficult to disrupt.
I'm a little unclear as to what the purpose of the bazaar is - the goal appears to be a state of perpetual violence that is capable of keeping a nation in a position of failure/collapse. That our enemies seek to use this strategy in Iraq is obvious, but success essentially means perpetual failure. What I'm unclear on is how they seek to parlay this result into a successful state (which I assume is their long term goal - perhaps that is not a wise assumption).
In any case, reading about the bazaar can be pretty scary, especially when news from Iraq seems to correllate well with the strategy. Of course, not every attack in Iraq correllates, but this strategy is supposedly new and relatively dynamic. It is constantly improving on itself. They are improvising new tactics and learning from them in an effort to further define this new method of warfare.
As one of the commenters on his site notes, it is tempting to claim that John Robb's analysis is essentially an instruction manual for a guerilla organization, but that misses the point. It's better to know where we are vulnerable before we discover that some weakness is being exploited.
One thing that Robb is a little short on is actual, concrete ways with which to fight the bazaar (there are some, and he has pointed out situations where U.S. forces attempted to thwart bazaar tactics, but such examples are not frequent). However, he still provides a valuable service in exposing security vulnerabilities. It seems appropriate that we adopt open source security techniques in order to fight an enemy that employs an open source platform. Vulnerabilities need to be exposed so that we may devise effective counter-measures.