I think the best reason for allowing knives on planes is that passengers are now conditioned to confront any attempt to commandeer the airplane. Prior to 9/11, conventional wisdom was that if you're caught in a hostage situation, you should not try to be a hero, and instead calmly get the plane on the ground, at which point the SAS or Delta Force or whoever it is can handle the situation professionally. That thinking went out the window once attackers no longer needed to land the plane. But now that people know that their plane might land, they'll just try to overpower anyone who does anything suspicious.
Chuck Schumer should let sleeping dogs lie on this one. Let people have their pocketknives; they're no longer a viable way to take over a plane.
Still waiting for the day I can leave my shoes on. At this point I'm guessing we'll have to wait until after Christmas travel in 2016.