The advantage is in eliminating the collateral structure in the tug itself, and placing its motive power in the wheel of the a/c. So we save carting around the frame and wheels of the tug. For this benefit we pay mightily for developing and certifying a small and exotic engine to carry around the sky.
They run off the APU power mostly; so no additional exotic engine needed I think.
Any airline that elects to carry 880lb of dead weight around is going to want to save a lot more than $200K in annual DOCs per aircraft.
880lb sounds about 4 lost PAX that could have flown. How much does an average seat bring in per-year on a typical short-haul fleet (with more landings / take offs I think that's the target market) these days?