Yeah, OP's pretty much a total failboat.
http://www.youtube.com/watch?v=skXQT8lrZe4
Not only does it prevent huge amounts of runoff, it cleans the air and removes CO2, provides a beautiful place for workers to take a break, helps reduce global warming (how many square miles of black tar roof are in the world, and what you think that energy absorption does?)....
You can argue that it doesn't make sense, but people have already proved that it does, in both economic and environmental terms.
OH, and yes you CAN grow things on your roof. People do it in highly populated areas all the time. Can you imagine the increase in air quality in a place like New York if this were done all over?