I agree that feel good mandates such as this demand greater scrutiny and shouldn't be taken at face value. I do,however, believe there is a place and a value to having green rooftops of all kinds in Denver commercial buildings and large residential units.

Vegetative rooftops are in a separate category from  PV rooftops. In addition to the listed advantages, which I believe(admittedly without supporting research) are sufficient justification, they also offer other advantages in an urban environment. They can provide a more efficient use of space and can play a part in the local food, parks and recreation movements.I particularly like them because they bring back some of the greenery and farms that we have otherwise lost forever.

Still, I believe that education( and research) and incentives are a far better way to go than a blunt, one size fits all mandate.