I'd guess that the vast majority of people who have Global Entry either paid for it using credit card benefits (most travel rewards cards have a Global Entry credit) or have it paid for by their workplace.

There are still marginal benefits of GE over Mobile Passport - more airports have GE, you get expedited service at land borders and at Canadian airports, and you can cut the standard line if the kiosks are broken. My favorite part of GE might be the reciprocal benefits you get in other countries though. For instance, you can sign up with the UK to use their automated border gates which is normally limited to EU citizens only.