My intuition is the terrorist planners expected a big crash and many killed, but not the complete collapse of both towers. The collapse of both towers has effectively started a War on Terrorism, of a magnitude they probably didn't expect.
Maybe I'm wrong. But I somehow feel the unintended consequence of the attack, will be a big blow to Islam. This religion has a lot of ground to make up. It is now tied to this event, and will be suspect for a very long time. What goes around, comes around.
Or in other words, that which they meant to advance, was set back.