Right or wrong, that is a tactic that indicates a weak argument. Just as when Jesse Ventura dismisses someone as naive, it doesn't help anyone get closer to the truth. Ridicule makes the facial case that your argument is weak, makes your opponent appear reasonable, and leaves you no more options.
Personally I don't know, and don't think it's terribly valuable to debate. Will governments stage provocative incidents to incite towards war? Even a cursory understanding of WW1 and WW2 shows not only will they, few governments didn't. Ours certainly did.
So whether 9/11 is such a case, and arguing over it, misses the point that governments shouldn't be in the position, and have to power, to engage in such activity at ALL.
To get rid of that power and desire, end central banking, and obey the Constitution. It's as simple as that.
Arguing over 9/11 was staged really doesn't help us much. I think it does help the bad guys more.