To add weight to what Robert and Pamela Geller are saying about the importance of the positioning of the GZ Mosque is in Arab translation of Imam Rauf's book that Robert points to:
In 2004, Rauf wrote a book entitled, according to the English translation, “What’s Right with America is What’s Right with Islam.” Such a title suggests a moderate perspective is taken. However, the book’s Arabic translation—“The Call From the WTC Rubble: Islamic Da’wah From the Heart of America Post-9/11”—suggests otherwise. Rauf, denigrating the loss of American life in calling it “rubble,” seeks to use 9/11 as a springboard for selling Islam to America. [+]
Imam Rauf has already said that the building was chosen because it was struck by some of the debris from the 9/11 attack, a part of the landing gear, travelling all 5 floors of the current building, to the basement. So from the 'rubble' of 9/11 comes Islamic proselytization. Imam Rauf help found and runs the Shari'a Index Project which measures Shari'a around the world. Would be well versed in Shari'a and would understand Muhammed's instruction to building a Rabat, an outpost for the spreading of Islam on or near the area of conquest. [in the old days this would have meant a place which to launch attacks from, which was equivalent to spreading Islam]. There is a city called Rabat in Morocco, for a long time the Atlas mountain people fought a resistance against the Islamic conquest. And today they are still in that fight, in part, they are all forced to give their children Islamic names, over traditional Berber ones or else the child's birth cannot be registered.