Just saw a brief yesterday by RADM Shannon talking about UCAVs tanking from manned hornets and KC-X.....I agree it would make much more sense to have an unmanned tanker do it instead.
You have likely never seen an experienced tanker driver hawk the recovery and then be exactly where it needed to be when an aircraft is in extremis. One thing many folks don't realize is Unmanned Aerial Systems (UAS) that include everything (from UCAVs to potential tankers you are talking about) don't remove the human from the equation quite yet. They simply put human somewhere else. In case of UCAVs, they remove human from going in potential harm's way and allow aircraft not to be limited in performance by what the human body cannot sustain and eliminates all the weight and volume required to host a human.
In the love affair with UAVs to provide ISR and more and more finish capability, the LIMFACs associated with having the UAV overhead, but the pilot and sensor operator halfway around the world have resulted in less than optimum results in some cases hence the trends toward more and more manned nontraditional ISR platforms like the Project Liberty MC-12 platforms and other examples that put the human overhead and on the deck where they can train and plan with the people they support.
Back to unmanned tankers in the sky. The movie stealth featured a mammoth unmanned tanker in the sky and AFRL has briefs on proposals almost exactly like the movie with addition an arsenal of net-enabled weapons (sort of an Arsenal Ship in the sky). For benign tanking, it makes sense to have as much gas as you can but in the sky, but for extremis situations that occasionally arise, I submit you still need someone in the cockpit who's the best you can get to be reacting and thinking way ahead of the recovery situation. Alas, one day they may not be an aviator in cockpit of the tanker or receiver.....