They need to control the aircraft in order to hit our most "sensitive" targets, right? Flight 93 was headed somewhere in DC.
They would need to gain control of the A/C to do anything, but once they did, its essentially one giant air to ground missile.
Thats 9-11 in a nutshell.
You know, they don't actually have to hit anything of any imortance. All they have to do is get us to shoot our own planes down. I don't know if that's easier than flying the plane into a landmark or not. Also, we've already considered, haven't we, the possibility of them sending a large number of terrorists on a mission so that some could do the suspicious behavior while others sit back & wait for the air marshalls & useful passengers to identify themselves.