I was sat on a 737 a few days ago recalling this thread and thinking...
Yes, I could quite easily bolt onto the flightdeck from here (Seat 5c). Of course, I didn't have a mind to do such a thing, but looking at the single, flimsy door behind the crew and imagining them strapped in the seats as a miscreat battered them over the heads with a bottle of duty-free, the crash-axe or simply their own hands doesn't bare thinking about. The stewardess strapped into her seat near the door for takeoff sure isn't going to be able to do much about it, if anyone tries it.
Which makes the new "jumpseat" rule less valuable than it perhaps sounds at first glance. I tend to agree about the value of El Al's "kissing gates". My understanding is that this would have prevented anyone trying the kind of attacks used on 9/11/01, admitedly they could still have slaughtered the passengers, but they're not likely to bother with that when they know they can't get to the flight deck to attack the flight crew and fly the plane into buildings.
As a frequent passenger and confirmed propellor-head, i'd have jumped (groan!) at the chance of a jump-seat ride. Looks like I won't get the chance now. Which is a crying shame.