It might not be economical *now*, but it will be when the economy is soaring. Also, the Twin Towers were landmarks. How can you not restore landmarks?
Also, a lot of people are mistaken...Twin Towers weren't hit b/c they were tall, but b/c it was the center of America's financial prosperity and what they stood for. While there are plenty of people who will be afraid to work high up in the building, there are many more who would love to work at the top.
What I say is...if they don't want such tall office towers, rebuild the twin towers to the same size. 50 stories will be occupied by offices and the other 60 can be empty space, or keep them closed and available for when people want to move back in. I'm a New Yorker and I saw them all the time. It hurts a lot not to see them there.