A fairly large number of people don't drive but have cellphones (Kids, NYC etc). So, I don't think numbers of people who turn it off is going to tell you much.
I turned it off because it kept disabling me when I was on the train (I drive about once quarterly). It's a good start, though.
You can have it only turn on when you connect to a car bluetooth device instead of using motion detection. It also applies if you have one of those bluetooth to auxiliary adapters if you have an older car.