I voted that it doesn't matter either way.
Even if, tomorrow morning, somehow incontrovertible evidence popped up displaying every truther group to be a fraud, there are a certain subset of individuals for whom the necessity to gravitate toward paranoid outlooks takes precedence. With the increase in communication over the internet and the unrest in the US and other nations since 9/11, these paranoid messages have become more convincing for more people. You're not going to change that.
On the other hand, I fully support pointing out the errors in wild and unrealistic claims made, not only by truthers or UFOlogists or the like, but by politicians or businesspeople or by those who are more personally known to you. It doesn't always have to be an argument, either, though sometimes it may come to that. I feel that we all need to be challenged more often on our preconcieved notions, even if we don't believe we have preconcieved notions. It keeps us intellectually fit.