I like the idea of a space tug that can refuel and move satellites in orbit, but this role seems to be at odds with bringing cargo to the ISS which is the goal of the CRS(2) contracts.
From what I understand the plan goes like this: On the first flight Jupiter (the tug) and Exoliner (the cargo vessel) go up together, once they are in orbit Jupiter adjusts the orbit to reach the ISS, after the cargo is offloaded and garbage is loaded Jupiter puts Exoliner on a path to burn up in the atmosphere while it itself stays in orbit to pick up the next Exoliner that's launched alone, as well as other tug duties.
So the problem as I see it is this:
For a tug you'd probably want a much more efficient ion drive to avoid refueling often, fuel boiling off and the like, you probably want the robotic arm that grabs on to wayward satellites.
For supply deliveries you probably want liquid engines because some of the supplies and experiments are perishable and can't afford to wait the weeks or months it would take an ion engine to boost them to ISS orbit.
And the grabby arm is redundant mass because the ISS has its own arm that's quite proficient at berthing other vehicles like the Dragon or Cygnus.
So it looks like a compromise of design that's intended to get NASA to pay with the cargo delivery contract for unrelated functionality.