Certain data can't be replaced. Society's overwhelming reliance on the information infrastructure, the electric grid that powers it, and in many cases the information itself can give the modern terrorist a far more effective target than does any individual. And it's a target rich environment. Some of these targets can affect a great many people indeed. Others expose the economy itself, and not just terrorists, either.
Common criminals can go high tech, too.
We know where your servers are. Of course, the modern terrorist need not go as high tech as E.M. weaponry to threaten the information infrastructure. There are certain key points in the power grid where a lone sniper, plinking ceramic insulators with a .22, could black out a whole group of states. Were the perp to take care not to leave any forensic evidence, and also keeps his or her mouth shut later, chances of capture would be virtually nil. A team of such snipers could black out the continent.