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.
Even if the motor is quiet you still have the propeller making a lot of noise. I (honestly) don't know if you can really get it quiet enough for the motor's noise to be important.
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.
A solar-powered sea-plane would be pretty awesome. Probably not enough range to cross an ocean in one go, but you just land, do some fishing, then take off again the next day.