Until September 11, nobody thought someone could bring down a plane with a blade.
...and now they can't.
It's now standard procedure to lock the cockpit doors, and pilots aren't going to respond to threads of "let us in or we kill this hostage", since they know a hijacking doesn't necessarily mean just a re-routing of the plane and possibly two or three dead people if demands aren't met.
They could let short swords onto planes now, and no one would be able to hijack a plane with one, because even several men armed with swords wouldn't be able to get into the cockpit.
(Um, note that I'm NOT recommending that they start allowing swords in the passenger compartment. I'm just sayin'...)
The only weak point is if the pilot or co-pilot needs to leave briefly to use the rest-room. But even then-- the only time I've seen that happen, there was a rather burly, bouncer-looking flight attendent standing gaurd the whole time. Even if he had been rushed by six men with swords during the brief times when the cockpit door was opening or closing, they still would have been able to get the cockpit door closed and locked again before the men got there.
In order for a passenger or group of passengers to hijack a plane now, they'd need a way to force open the cock-pit door, while the flight attendants and passengers are trying to stop stop as though their lives depended on it (because they know they probably do.) Not to mention that when there is any sort of disturbance on a plane, it is now escorted to the nearest airport by U.S. Airforce fighter jets.
If there's another terrorist attack or attempt thereof, it's NOT going to be in the form of passengers hijacking a plane.