You can launch the multicopters from places where you're not seen. You can even do this in nighttime and have them land and wait undetected on some rooftop until you want them to continue their mission.
A handfull of such multicopters will make more terror than 9/11 ever could do because 9/11 was (almost) concentrated to a single place. 9/11 had passenger lists so it was easy to find out who took control of the planes. Multicopter bombers have no passenger lists, no sender address. You don't know how many more planes are in the air and when they will strike.
Redundancy and multi-target missions are even cheaper and more efficient with multicopters than in the 9/11 attacks. Multicopters are so cheap that the terrorists can launch a bunch of them and not care if half of them fail to get to their target. Multicopters bombers works for targeting e.g. Disney Parks and other places with a lot of easily frightened people out in the open.
With 9/11 it was easy to get away from the attacks (i.e. just leave that part of Manhatten). Multicopter bombers are like a swarm of supersized wasps. You don't know where to go, you don't know when they'll be back.
1: Homemade "grenades": http://en.wikipedia.org/wiki/David_Copeland