Disagree. Knives worked spectacularly well on 9/11.
Yep -- and muskets worked spectacularly well in the War of 1812.
This is the problem with deterence - you may never know what you stopped. Sometimes you will, but mostly you will not. And you'll never get the press coverage that a horrific incident would have.
The average screener won't know and probably nobody at an airport will know. In fact, if a real terrorist bust was going down at an airport, need-to-know would be very restricted. And, it wouldn't be the TSA making the bust.
An airport checkpoint is like my home burgler alarm system. The only bad guys you're deterring are the amateurs. But, this isn't the strategic deterrence business, it's public relations. If the TSA actually caught a real terrorist, I'd bet my mortgage it would be all over the news, and everyone from Bush on down would be taking credit for it.
If you've got access to the TTIC daily reports, read them and see all the amateurs we're catching.