The quiet aspect probably has a lot of value in military applications. And it'd be useful in the solar powered unmanned planes we see, but maybe the energy requirements for this are too high.
http://en.wikipedia.org/wiki/Lockheed_YO-3
The largest source of propeller noise is transonic flow at the tips, so turning the propeller slowly helps significantly. The tradeoff is a larger propeller and more expensive engine installation due to the required gearbox.