The thing is that if you are able to refuel a transport while still in the air, you can technically carry more cargo than if you didn't as you come up against the maximum take off weight of the aircraft (airframe plus fuel and cargo), as the airplane has only enough fuel to get off the ground and rendezvous with the tanker to get a full tank of gas to go where it is needed. We see this problem all the time with airplanes; the more cargo you want to lift, the shorter the distance you can travel as you are forced to take less gas due to the fact that you have to remain under the maximum take off weight of the aircraft otherwise, one the following conditions could happen:
1. Airplane doesn't get off the ground period (airplane intact and crew is safe)
2. Airplane doesn't get off the ground period, and crashes into an obstacle at the end of the runway (airplane not intact and crew in danger)
3. Airplane does get off the ground but by chance, suffers an engine failure, and the airplane plummets back to terra firma as the airplane can no maintain V1 speed, which is the speed where an engine failure can occur, and you can still take off, and usually by the time you reach V1, you are already committed to taking off (airplane not intact and crew in danger)
We have already seen this happen in a crisis situation before: The Yom Kippur War in 1973. During the war, the Americans were flying C-5 Galaxy's to Israel with only a fraction of of their maximum payload on direct flights from the continental United States to Israel due to the fact that the C-5's were denied landing rights in Europe. The Americans, as a lesson, firstly trained their C-5 Galaxy crews in aerial-refueling, then after that, recognizing that a more capable tanker was needed, purchased the KC-10 Extender tanker.