If the Department of Homeland Security tells us there's a good chance of terror attacks, and then nothing happens, they can claim that they've done their job in keeping the country safe. How would we know if they've really thwarted terrorism?
Let's suppose their claim is accurate. What are are we, the public, going to do about it? Be on the lookout for suspicious activity? Stay at home and pray we don't get blown up? All they're doing is fueling fear.