Ok, someone who remembers correct me if I'm wrong. There are different kinds of Marshalls. During the 1970s a uniformed armed Marshall was placed outside the cockpit door during the entire flight. The guns they carried were specifically designed NOT to breach the hull of the plane or in any way threaten the integrity of the aircraft. In addition to that there were undercover armed Marshalls throughout the plane. (EL AL rewrote the book on it. I can't say that I've ever spotted one of them on board & on long flights its something to amuse myself with when I have nothing else to do)

So what if we are able to spot the plain clothes Marshalls? Does that make them any less effective? Public awareness of their presence makes them a deterent to hijacking. However, their function is to protect us, the passengers in case there is an incident. I don't know about anyone else but as long as they're on board, I don't care what they are wearing.