The assertion was that it's demonstrably useful for national security. Without such a demonstration, I submit that it is not, and that terrorists are caught using normal police work.

"It's too secret to tell the public in broad terms a decade after the investigation is over" strikes me as incompatible with democracy. Questionably-legal surveillance also strikes me as incompatible with democracy outside of short-term use in exceptional emergency conditions, which should be disclosed to the public once the emergency is over.