I think that the point of terrorism is fear itself - hence the name terrorism.

If they've gotten the population to behave in a fearful manner, they've accomplished their goals, regardless of economic impact, loss of life, etc. The whole point is to elicit an emotional response and establish (psychological) dominance.

I don't necessarily think that prudent security measures are a bad thing, but some aspects of post 9/11 security like the "see something, say something" program or the security theater at airports are absolutely playing into the terrorist's hands. Good security is generally invisible.