I'm not sure on this one. Reasons for my doubt:
* so hot and windy up there
* easy to forget
* anti- gravity to get water up there = energy (not good in peak oil scenario)
* rooves need to be strong to hold lots of soil/ water tanks - expensive to retro-fit
* such a nice place for solar panels and solar hot water systems
* more land should be available for growing food in cities once the car is no longer king (which should be reasonably soon) - asphalt is the enemy!!
On the other hand:
* good for thermal regulation of buildings
* un-used space
* very secure (mad max connotations there :)
What do you think?