It is far more likely that an Air Marshall-pig would go crazy and shoot up the plane, or have an accident with his gun, than that any actual 'terrorism' would occur.
Ah yes, I forgot to read up on "odds of Air Marshal going on a crazy rampage Vs. Likeliness of insane/criminal/terrorist activity on board a plane"
Or simply accidentally shooting off his gun. The sort of people who go in to law enforcement are hazardous both because of personality disorders and general stupidity.
Guns have a SAFETY switch for this reason, and they wont be fiddling with the gun anyway... I'll also point out that all Air Marshall guns have reduced powder rounds to avoid exit wounds and/or damage to the plane hull...
If it came down to it, I'd choose to have an Air Marshall on board than to not have one