I'd wondered the same thing so your question inspired me to do what research I could on the internet.
Took some digging but came upon a couple of enlightening discussion threads:
Electronic shutter vs mechanical shutter: Nikon 1 System Talk Forum: Digital Photography Review ¿Why don't EVIL cameras have silent shutters?: Open Talk Forum: Digital Photography Review
From what some posters indicate using the electronic shutter may negatively affect the image quality by making the sensor more prone to light blooming during the exposure.
In my experience the MX1, even with electronic shutter enabled, will cap the shutter speed at 1/2000 unless you bump it higher in Tv or M. If that's the case then it's probably to preserve the image quality.
Still digesting all the other trivia I picked up along the way.
Anyone with more knowledge got any insight?