Yeah-the Rosenberg execution had nothing to do with national security; it was part of trying to destroy the political movements of the Thirties. If you want to traumatize people, treason trials are a n extreme way--if there are spies running around in our midst, then we're really in trouble, we'd better just listen to the government and stop thinking.
Look, every government has a need to frighten its population, and one way of doing that is to shroud its workings in mystery. The idea that a govermnet has to be shrouded in mystery is something that goes back to Herodotus [ancient Greek historian]. You read Herodotus, and he describes how the Medes and others won their freedom by struggle, and then they lost their freedom when the institution of royalty was invented to create a cloake of mystery around power. 30 See, the idea behind royalty was that there's this other species of individuals who are beyond the norm and who the people are not supposed to understand. That's the standard way you cloak and protect power: you make it look mysterious and secret, above the ordinary person- otherwise why should anybody accept it? Well, they're willing to accept it out of fear that some great enemies are about to destroy them, and because of that they'll cede their autthority to the Lord, or the King, or the President or something, just to protect themselves. That's the way governments work--that's the way any system of power works--and the secrecy system is part of it.
Source: Understanding Power.