Good point. In addition, they have stated that their goal is to bankrupt the USA just like they helped bankrupt the Soviet Union. And, by tying up our forces in Afghanistan, Somalia, the Philippines, and by us going into Iraq and letting them in, they are getting just what they want...
However, while others had thought of using planes as weapons before 9/11 (Tom Clancy, Dale Brown, Thomas Harris, "Max Barclay" (Novelists), Various Japanese, German, Russian and American pilots (Including Joseph Kennedy, Jr. and Charles J. Loring, Jr.(MOH Korea)) (pilots/military), the Tamil Tigers, the GIA and Samuel Byck), Al Quaida was the first to actually use an airliner as a weapon, thus getting the credit. If they thought outside the box yet again, they could cause a great deal of economic damage, and frighten the american people. (As one of the Al Quaida members put it, in his plans for something like 9/11, "Boom, Boom, Boom, and America is on standby.")