If it is pretty close to 1.0 efficiency, or at least more efficient that conversion of other intermittent energy sources (like solar, wind or unicorn farts) into battery power, hydrostorage (pump water to the top of a dam during high production times to let it flow down through turbines during high use times), hydrogen fuel cell/electrolysis systems, etc., it could be useful. It could also be good to allow power sources which run best at constant output, like nuclear reactors, to do something useful with the energy during the night.
Sounds like you would end up with a huge net loss of energy doing that. The energy used to pump the water up would far exceed the energy you would recover from the down flow through a turban.
It just doesn't add up to me.