I think you'd lose a bit of efficiency have to pump circulated seawater to the roof tops to not only provide water to desalinate, but also to flush out the accumulated brine overnight.
Those skyscrapers teem with occupants, each with a bladder, each one a source of saline. The spare water and power we use for growing cucumbers in the hydroponic gardens. The waste brine we use for picking those gherkins, which makes the ideal beer-time snack for those occupants as they re-fill their bladders...
