Seriously though, consider the false negative rate they have (by some accounts, as high as 70%), and the cost associated with actually being caught (they throw your knife into a bin, then wave you through onto the airplane). It just isn't plausible that the TSA is making terrorists to afraid to try.
Either the terrorists just don't care about hijacking the planes anymore (fairly plausible, planting IEDs is probably more fun), or something else is stopping them. Perhaps the thought of being dog-piled the second the passengers figure out what's up...
How are you evaluating benefit? We haven't had a major terror attack using planes since the TSA was brought into airports.
(yes, I'm playing devil's advocate here, but only because that's exactly what would happen)