The main benefit of having a mechanical shutter is that it hugely reduces and simplifies the circuitry on a device. Having an electronic shutter (where the sensor basically starts gathering light for a set period of time) means added electronics at each pixel location.
The mechanical shutter on the other hand often allows for a simpler and more effective sensor with a higher fill rate where each pixel gathers more light.
All of this along with the rumored large sensor would make not just for a great all-around cameraphone, but for one that performs particularly well in low-light. Those are all guesses right now, though, so take a look at the video below to see the Nokia EOS camera in action.