The point is that it would increase the demand by the US public and Dems in Congress to withdraw our troops NOW!
Obviously running away from Al Qaeda is the best defense (/sarcasm).
All AQ has to do is kill a few hostages, infect a few soldiers with some diseases and the US public obligingly surrenders and converts to Islam. Bin Laden is still operating on the evidence of their attack in Somalia -- a few people die, US obligingly runs home with its tail between its legs.
President Bush isn't going to do it, but if there is sufficient public outcry, he may not have a choice. AQ are fighting the public opinion war, where with the Dems help, they are gaining.
Al Qaeda is not gaining, we are winning this war on terror and will eventually achieve the ultimate victory. Stop your stupid pessimism.