Well in the last few weeks, 'thick coppers' with guns have satisfactoritly concluded:
1. An El Al 'incident' into Turkey
2. A Royal Jordanian 'incident' into AUH
I think it quite reasonable to conclude that both these incidents would have been far more serious and inconvenient without the Sky Marshalls presence. There seems to be a horror from some on Pprune of "I don't want guns blazing on my aeroplane!". Let's think about this- these Sky Marshalls are well trained not to discharge their weapons without good cause. It would be because hijackers either have guns or other serious weaponry already . If so, you are already likely to be walking dead soon unless the Sky Marshall stops them, so as far as I am concerned, "Welcome Aboard, S. M! And don't refrain from drilling as many holes in skyjackers as you like because I know what the alternative is!"