I posted this one here last year - I have used it a few times to point out that 911 indeed SAVED the struggling US economy after the dot-com bust earlier in the year and Iran/Iraq/Saudi all about to drop the dollar.
Thanks to that trillion dollar hit to the economy, we've been able to borrow nonstop, use govt subsidies to save any inefficient business in the country, make haliburton rich beyond belief, and yes, force Iraq and others to keep trading with us (at incredibly low rates!!) when they didn't want to deal with the great satan anymore.
911 will put the US economy in MUCH better shape in the coming decades than had it never happened.
This of course, is motive, for those who do understand the numbers behind it. And the reason we should have blown up something unmanned instead of 3000 civilians.