I'm just saying that if you put a guy with a gun in the back, the hijackers will simply build a plan that neutralizes that guy.
For instance, the four hijacked planes each had about 70 people on board, when they were capable of carrying 200+ Coincidence? I doubt it. I'll bet money that the demographics of airline travel was analyzed and it was determined that a passenger load of that size was "controllable" by a team of 4 or 5 knife-wielding attackers, one of whom would later be busy flying the jet, and that those flights were picked precisely BECAUSE of the size of the passenger load.
Throw a possible armed defender into the mix, and you simply force the hijackers to use a bigger team, with perhaps a diversion set up to draw out the skymarshal so he can be ambushed while he's busy killing the decoy (remember, these guys all KNOW they're going to die when they get on the plane--dying taking out the skycop would probably be a "plum" assignment)

And NOW you have the hijackers armed with a captured weapon that WILL go through the (currently unreinforced) cockpit door.
It's an outdated response to last year's hijacker. The hijacker of the future wants the airplane, not the hostages, and if he's going to "martyr" himself, he's not going to be squeamish about losing some of his team if it means getting a weapon with which he can control the crowd. So he uses 10 guys instead of 4, and runs the cop out of bullets. He still wins.
The plan that worked on Tuesday would still have worked with skymarshals on board--it would have only required dealing with the skymarshal first. The only thing that would have defeated the plan (other than having useful intelligence before the fact) was what happened in Pennsylvania--physical resistance by the passengers.
What would have prevented it? A 180-degree change in the standard response to hijacking: refusal to come out of the cockpit, immediate landing, and acceptance of passenger and cabin crew casualties as "inevitable."
A bitter pill, but there it is. Ironically, the system for preventing "real" weapons on the planes seems to have worked: the hijackers didn't risk trying to get firearms or explosives on board--they HAD to get on the plane safely or their plan was doomed. What is tragic is that they were able able to kill thousands while wielding household tools, simply because our aviation mindset says "try to talk them down."
Skymarshals will just be heroic targets, killing terrorists who planned on dying anyway, and they'll just provide hijackers with weapons they couldn't get on the plane themselves.
The solutions to this problem are on the ground and FORWARD of the cockpit bulkhead, not in the cabin.
Just MHO.