By standing out they definitely kêp the plane they are on safe. Terrorists are not going to take on extra trouble if they want to hijack a plane.
Indeed. I've been told that on an El Al flight they wear a uniform. Seems to work for them. Also, the air marshals can expect the assistance of everyone on the plane who has a brain.
4-5 hijackers vs 2 armed men+ flight crew(with taser) + the able-bodied passengers + Threepwood!:) = It should be ok.