I assume that if a flight has air marshalls, it will have only one. What if you had a lone "nut" freak out and attack the air marshall to distract him, while the accomplices used the diversion to "neutralize" the air marshall while he was distracted? If the air marshall is busy trying to control the unruly passenger, it might be easy to get the drop on him.
Just a thought...thanks for the bandwith.
More to the point, Air Marshalls travel "undercover". Staging an "ordinary" disturbance which would get the Air Marshall involved would allow the terrorists to identify the Air Marshall so that they could then neutralize him unexpectedly later in the flight (or as you point out, while he's distracted by the original disturbance and focusing on that).