No sir not saying it is OK to just build over it. I am personally in the camp of "rebuild even bigger in defiance".
However, that is not all that is happening. They are building an awesome memorial park for reflection. The original footprints of the towers will become reflecting pools, and alot of the rubble and rooms will remain intact.
http://www.projectrebirth.org/rebuild/architecture/theMemorial.html
I guess my thought is, in NYC, an amazing US metropolis, that prime real estate WILL sooner or later get redeveloped, much like I mentioned has happened in Hiroshima and Nagasaki. To me, why let it sit as an empty hole long enough to be forgotten and made into an office building? Why not claim it now? However, these are just my opinions.