With all the conspiracies, the only motive was they were trying to make the Taliban & Al Qaeda seem more powerful and more strong then the America public believed them to be. If so, they've done well - people are still scared in case an "anniversary" attack happens, which is clearly unlikely to impossible, considering new security and heightened security for the day.
The weird thing is, in Vietnam the American's pulled out, due to public anger at the war - and how they were fighting in Vietnam for no real reason. Could this have been why the towers was an inside job? Did the American Government not want another civil up-rise over the war?
Food for though. That's my only real conclusion as to why they'd bomb two of the largest buildings in the world - which cost billions to re-create.