Using "milch cows" seems like a cost effective solution to fueling/supplying a moon run. Of course, the green-eyeshade crews will get to have the last say.

It's not as advantageous as you'd expect, because your ships now have to have docking collars extra sensors, and extra maneuvering capability. The rendezvous will take extra time and fuel, and the extra time means you need more life support for the crew.

Worse, the extra steps involved create extra risk, both for mission failure and to the lives of the crew. You could get to your fuel depot and discover the battery sytem has failed and your depot is tumbling, or a leak caused ice buildup that prevents you from docking. Or the chillers failed and enough oxydizer got vented into space that you can't continue the mission.