Here is a sensible article about conspiracy theories on a far-right, racialist website:

http://www.toqonline.com/blog/cranks-and-credibility

Interestingly, the author makes the same allegation against 9/11 truthers, birthers, and other nuts, as Alexander Cockburn does on the left:

"Personally, I believe many in our movement diminish the credibility of our fundamental insight by indulging in conspiracy theories and wild speculation"

The point is not that holding up signs saying "Controlled demolition" and "Israel did it" are harmful to the politics of left or right or anything, it's that they are useful to the establishment by making its opponents look ridiculouts.