So you are saying that it is there to ensure mission success because of who is at the controls and because they might have been nervous and/or be potentially incapacitated by others?
If I'm not mistaken, the story has it that someone did intervene on United 93, or is that story false too? Well, I'm willing to believe some dramatization there with proper evidence, but in any case, the motivations and tactics used by the hijackers in the OS at the two WTC were simplistic and relatively easy to execute by comparison to modifying an aircraft behind someone's back. In taking over AA11 and UA175, passengers would have had no idea what was happening until the final moments other than a rather violent and swift move by hijackers. The nerves of the pilots? Probably shot but that doesn't mean they are going to just cower and turn back. Keep in mind they had a whole team likely supporting each other for the mission. Besides, crashing into a building is a pretty quick way to go, and there are people out there who are not afraid to die.
And really, have you even considered the complexity of installing some super-high-tech device to takeover a complex jetliner like the 767 behind someone's back? This is what the panel looks like:
And this is how complex the innards can get:
This isn't as simple as they do it in some sci-fi movie here with some ice breaking gizmo.
The only thing that is simple in the equation is how they had reportedly executed their plan. If I were the plotter, I'd keep it simple.