I can see both sides of the argument....
Speaking from a point of virtually no knowledge of what actually does happen on a plane..
It does seem to me that this is the good old "More guns will make it better" school of thought. Adding additional guns to a situation isn't going to make it better in most cases. What we really, really, really need is tighter airport security. But adding armed guards to the aeroplanes does make for some exciting "getting tough of terrorism" headlines....... Simply annoying passengers with more thorough ID and clothing checks isn't going to curry many votes, is it? Or am I being excessively cynical?
I don't know how effective these guys would actually be. The terrorists will doubtless start to arrive mob-handed and then we'll need more marshals to protect a plane. It more-or-less assumes that the air-marshal isn't going to get himself killed (which is what is likely to happen if there is a 6-on-1 shootout!) and is going to realise and confirm what's happening before it's too late. Once the terrorist has burst onto the flightdeck, there is probably precious little anyone can do. It's not a movie, there is no script and virtually no rules. The terrorists are not that stupid, and will anticipate you being there. It's not going to be a turkey-shoot and it will never be obvious who the enemy actually is. Ununiformed people is an idea, but not foolproof. Terrorists will spot them easily be seeing who *doesn't* go through security? There would be powerfull arguments for sticking them in uniforms.
I suppose we have to do something, but if we do we want policemen, not soldiers, on those planes.