Not all of the resorts have them yet. The last time I was there, in Nov, the resorts that had them were in the minority. I'm sure they'll be adding resorts as time passes.
I actually prefer the monitors at the bus stops to an app. Easier to judge if you'll get on the bus. As someone already said....everyone looks at their app at 8:20 and sees that the MK bus will be there at 8:28, and they all head to the bus stop! Huge traffic jam!
In my very humble opinion....too many people are worrying about bus rides taking an addtl 15 mins! This is vacation....slow down, smell the roses!!! If you feel you really need to be able to get from point A to point B in the quickest way possible, then you may be better off renting a car, or taking a cab.
I just can't imagine that any app Disney could come up with, for transportation, would be satisfactory in the long run.