Green roofs are becoming more common. I don't claim to know much about them, but obviously buildings can be built economically to hold the load. I have seen York University's Computer Sciences building's green roof. It's planted with wild grasses mostly, and maintenance is almost nil. Most of these roofs are for environmental and energy-reduction purposes, not for human ideals of beauty.
Here's a very readable article:
Come to think of it, since Toronto stopped herbicide spraying public areas I've noticed a remarkable increase in butterflies ~ and no doubt the green roofs are contributing to balancing their habitat as well.