Green roofs are always touted for their ability to insulate and moderate water run off.  This is only the most recent manifestation of everone's enthusiasm for them.

Green roofs involve a soil-like substrate, and plants.  The plants are green, and are a living symbol of nature and sustainability.  But what percentage of green roof benefits come from the plants, and how much from that soil substrate?

I've never done a formal study, but several green roof vendors have told me that most of it is from the substrate.  Plants can transpire water, making the substrate dry more quickly after being soaked by rain, but they don't store much. Almost all of that "minimized runoff" is due to that substrate.

How appealing would it be if someone said "hey, we want to strap a giant sponge to your roof?"  You don't have to be a marketing expert to recognize that that is a nonstarter.  It would make no sense at all, and no amount of argument about runoff or cooling in the summer would be persuasive.

Put high-maintenance plants into the substrate, and it is suddenly something appealing, except to anyone who really needs to manage repairs for a large commercial structure. Particularly since what you gain in summer insulation, you lose in the winter. As one vendor said "Are you warmer in a wet coat?" You can get lots of additional heat loss.

I'd like to see a green roof and a sponge roof compared side by side.  I think the results would be enlightening.