When I asked that question a while back, someone in the know advised that there was too much latency in the cellular system. To me it is a great idea...if it worked. In concept, as long as your cell has service and your bird is within cell service range, you can effectively control your bird from anywhere. Pointing your controller's antennas toward your bird would be a thing of the past.
Still, I would love DJI to be working on that. Disconnections and poor video downlink drive me up the wall. I guess we have to rely on Lightbridge and Occusync for the foreseeable future.