Arlington:

The one that springs immediately to mind is allow the terrorists to succeed by not interfering with them, a low risk strategy with a high probability of the desired outcome. To add in all the other players and some kind of Plan B would add some benefits, but not nearly enough to justify the risk of discovery or something going wrong.

Totally agreed. Obviously the Truthers aren't restricting themselves to the sort of evidence that would stand in court -- which is why your argument leaves them cold -- but motivation is an important question to people who know better.

And the hypothetical conspirators (even assuming they had the ability, which is doubtful) did not have any motivation to double up on WTC-destruction measures. If all they wanted was an excuse to invade Afghanistan and Iraq, the planes crashing into the buildings were plenty excuse enough. The building collapse was not necessary to piss off Americans to the point of supporting retaliation. They would have supported war if the gutted buildings still stood while the corpses of jumpers littered the streets below and everyone above the impact points was burned and smoked to oblivion. The theorized "controlled demolition" would have added increased risk of being caught, but zero-to-little additional public-outrage value.

And it wouldn't just have added a little increased risk of being caught. It would have added a lot. It would be much easier for a small number of conspirators to disable a few key security measures while a group of zealots (who's willingness to die for the cause indicates a sincerity unlikely to have been inspired by, say, their love for George Bush) fly the planes into the buildings. To add the unnecessary step of sneaking demolition crews into the WTC, installing the necessary explosives, firing a missile into the Pentagon, crashing a fourth plane into a field (accomplishing nothing!) ... I mean, WHY BOTHER?! You had us at "planeloads of screaming passengers crash into World Trade Towers, killing thousands."

Finally, and in the same vein, why create a pretext for war that necessarily focused on Afghanistan only, when the goal was wars in BOTH Afghanistan and Iraq? Why not choose a conspiracy that would definitively link Iraq to an attack on the US (or at least manufacture some _convincing_ links between Al Qaeda and Saddam -- why is this part of the conspiracy so flimsy while the rest is so stinking efficient and perfectly executed)? Though neocons surely began hoping for war with Iraq on September 12, it was not a given in 2001 that outrage over 9/11 would lead to war with Iraq. Moreover, I think it's pretty clear that public opposition since 2003 has created a political climate that is now hostile to continued American war/wars in the Middle East. Surely, a more Iraq-centric conspiracy would have been a better pretext for perpetual war. For that matter, if the sky's the limit with conspiracies, why not create one that implicates the entire Arab League, Iran, Kim Jong Il, Mugabe AND Al Gore -- thus providing pretext for war with all our enemies at once)?

The problem for Truthers is that they can offer no convincing motivation.

You know what the real conspiracy is? It's this. It's "government is all-powerful" memes that perpetuate the myth that someone, somewhere, is in control.