Because I've hung out with lots of skeptics, I know a few basic facts about 9/11 truthers. I know that the movement only became popular around 2006 or so, due to the documentary Loose Change. I know that it is now declining in popularity after Obama's election. I know that there is a difference between LIHOP (let it happen on purpose) and MIHOP (made it happen on purpose) truthers.
I know that they make at least a few ridiculous arguments. In particular, I remember there was a firefighter on TV who said "pull it", referring to pulling the firefighters out of the building. Truthers argue that "pull it" is controlled-demolition terminology, and that this slip of the tongue gives the conspiracy away. Probably truthers have less ridiculous arguments, but this is the sort of stuff that we like to laugh about.
I know that I don't have the ability to argue against 9/11 truthers.
9/11 conspiracy theories are too specialized. You really need to do your research to argue effectively. It's all very well to look at the surface and mock it for fun. Or you can listen to arguments and point out fallacies. But to understanding how to recognize good and bad arguments can only get you so far; what you really need is knowledge. Without knowledge, the best I can do is defer to other sources.
I think of 9/11 conspiracy theories as belonging to skeptical arcana. Obscure knowledge is required. Unless you know your stuff, you can't argue about it. Your opponent is likely to know more than you, because they're more invested in it.
What other things qualify as skeptical arcana? Pretty much all conspiracy theories. Cryptozoology.* UFOlogy. Those crank theories that people often mass-e-mail out to physics faculty. Denial of anthropogenic climate change.
*People often deride bigfoot skepticism, because bigfoot seems so hokey and ridiculous, it would be pointless back-patting to debunk it. However, I think the ridiculousness of it means that only the most devoted people believe in cryptozoology; they're likely to know much more about the topic than you do.
What doesn't qualify as skeptical arcana? Creationism is one example. Most people with a college education understand enough evolutionary theory to rebut Creationist arguments. And anyone can argue against Biblical authority.
Skeptical arcana is frustrating for many armchair skeptics like myself. I don't have the knowledge to talk about it, and I'm not willing to put in the effort to research every little thing. But it is better to recognize this limitation rather than deny it. I believe the best solution is an institutional one. We have a community of skeptics, and just a few people need to be experts on any given subject. Those experts do the research and present the arguments, and the rest of us use them as a reference.