Suzanne just said, "There are no combo arc fault 2 pole breakers out right now" so therefore you can not meet the 2008 NEC requirement for AF breakers with MWBC's. So the guys in class all started to say that they would run the 3 wire circuit in order to get around that code section. The key was "COMBO AF breakers", the breakers that detect both series and parallel AF circuits.
