Maybe some actual airline pilots are willing to weigh in on this, but I doubt that Channel 9 poses a security risk to anyone. The 9/11 story sounds like baloney. If you're a "bad guy" then you can use a radio to pick up the conversation anyway, on the ground or in the air, no? What privileged information is being expressed??

I love Channel 9, even when it has scared the pants off of me. (Two incidents recently: one when I heard an ATC yell "United #xyz, increase altitude 3000 feet. NOW !!!!!" A little too abrupt for my taste... shortly thereafter I saw a company Airbus 320 in very close range, coming the opposite way, the closest I've ever seen another plane at cruising altitude... The other incident was on the way into ORD, when Chicago Approach lost contact with our flight. The ATC asked if any other planes could hear or understand us -- no. They eventually switched to another radio, but for a few minutes, it was indeed rather disconcerting. But the issues were FIXED and in a timely manner.)

I'm hoping that Channel 9 will be on on my upcoming trip into FRA. I'm curious to hear the German ATCs and how their "rhythm" differs from the folks in American airspace.