Well, apparently, the idea that 9/11 was orchestrated by the government is 'gaining momentum'. Max Horkheimer warned that ideas become instrumentalized in industrial capitalist countries. In other words, they cease to have value in themselves, and start being valuable in-so-far as they are useful. I find Horkheimer to be painfully gloomy, but it's an interesting idea when we talk about ideas 'gaining momentum' as if they were advertising campaigns.
In some ways, I think we're better off with 'conspiracy theories'. The alternative is to decide that some ideas are simply verboten. In the case of the WTC attacks, why shouldn't people wonder how two planes can bring down a building? Much less the one next to it? Part of the answer is that skyscrapers are more vulnerable than we would like to believe. But, I think it would be useful for independent bodies to research the mechanics of what happened.
The problem with a conspiracy theory is that there aren't really any independent bodies. Conspiracy theories take on the character of blind faith, and have come to replace faith in our secular world, because they have two postulates to them:
1) A crime was committed,
2) There is a sweeping conspiracy to cover it up, which could include anyone or no one.
It is the second statement that makes them blind faith because it negates the first.
I've noted on here before that I agree with Karl Popper's idea that what makes a scientific theory is its falsifiability. Popper argued that in so far as a theory is falsifiable, it is scientific; and in so far as it is not falsifiable, it is an article of faith. For example, we could never really perform an experiment that would prove that there is no God to the believer. But, for even basic scientific ideas, we have to be able to potentially disprove them, or they become closed systems of belief.
There have been numerous conspiracies throughout history. And, even given falsifiability, there have been plenty of close-minded scientists. But, the way that scientific knowledge proceeds seems to me to be based on the sort of open-minded research that Popper suggests. The problem with conspiracy theories is that they cannot be tested scientifically because they contain the second postulate. If I exhaustively research the claim that the crime was committed (here that a group of people demolished the WTC from the inside), and I find that a crime was not committed, the conspiracy theorist simply has to declare me part of the conspiracy. In other words, the second postulate can prevent nearly anyone from disproving the first.
The conspiracy becomes something like Satan. For the devout religious believer, God cannot be disproven, nor can the most unlikely parts of their Bible, because any proof that could seem to disprove it (say dinosaur bones) is the work of the Devil. Satan wants to tempt us, he wants us to disbelieve, in the same way that the conspiracy wants us to disbelieve. So anyone who argues against God is either seduced by the Devil, if we want to be charitable, or Satan himself, if we don't. Similarly, if I believe that we really landed on the moon, I am either a dupe or part of the conspiracy. No evidence that I can marshall can save me from that.
So, ideally, we should have no sacred cows at all. We should be able to explore these theories, but also able to reject them, if they are wrong. The reason I think that they give a sort of security to those who believe them is that they sense that their theory cannot be disproven, and therefore they can hold on to it forever. Any form of skepticism can become an equal form of blind faith.