^^But gravity is an extremely weak force, and a large body still possesses considerable inertia. A subspace field to reduce the mass of the tractored object would make it easier for a coherent graviton beam to accelerate.
I RL you'd be right. However, deck plates thoughout a ship provide 'artificial gravity' supposedly though graviton manipulation, and they can provide 1 g (and much higher, if IAMD is any indication).
Even so, if there's a means to reduce the amount of energy you need to exert to tow a starship, why not use it? If the means to reduce a ship's inertial mass exists, then it would obviously make tractors more energy-efficient if you could include that function (assuming it took less power than the raw towing would).