Horizontal organizations can't pose enough of a sustained military or, transposing the tactics, guerilla or terrorist pace of operations similar to "Tet".
If you have no leadership and bunches of "cells" charged with carrying out missions or objectives co-ordination becomes nightmarish. Think of it as a deadly game of "telephone". Without some kind of centralized command structure objectives and missions are bound to fail due to coordination.
Lastly Tet had two advantages AQ does not have. 1. A period of truce. Tet was supposed to be a lay down of arms and caught us off guard. No such truce would ever be observed in this war. 2. There are no major state powers supplying arms en masse to AQ. Furthermore if there were there would be no political worries about sinking ships or shooting down planes of foreign countries supplying such arms. Sure, the usual leftist suspects would decry the damage we did to our foreign image if we were downing Iranian transports but the world would collectively yawn.
I agree that a highly decentralized Al Qaeda can not fight us in open battle, especially after they were solidly destroyed in Fallujah. They will concentrate on "media events" conducted by small groups that are highly integrated and self-sufficient and operationally autonomous. All the new jihadists want to be the next Atta, not the next bin Laden.
There will be no coordination with other cells once the fighting cell is funded and pointed to a task. Most of al Qaeda will not know what is going on until it's on CNN.
The key to the fighting cells success depends on the talent of the "key person" and the logistical support the cell receives on start-up, i.e., whether or not the cell is truly self-sufficient through the entire operation.
The logistical build-up to Tet occurred well before the truce, all the truce did was allow freedom to stage troops. There is a working truce of sorts, a disengagement, in the tribal areas of Pakistan, that is allowing the same thing.
Lastly, the supply of battlefield arms is now largely irrelevant and needed mainly for defensive purposes by al Qaeda. The new weapons will be acquired at the task location as needed, freeing up the jihadists' movement and concealment.
One jihadist, a van full of gasoline, one road flare, on a crowded Staten Island ferry full of gassed up vehicles all packed together is all that is needed for the next CNN moment. It is not that hard to figure out any number of easy world-wide targets that can be attacked with characteristic al Qaeda low tech weapons.