If I remember correctly, 'King's Gambit' involved two intersystem jumps. So the intrasystem drive would've been useless.
As for subspace, I think it's a little inconsistant. For example, if you were inside a Black Hole's gravity well, and you made an intrasystem jump away from the Event Horizon, you woudl use a certain amount of energy X. If you were to move the same distance on fusion drive, you would use a (probably greater) amount of energy Y. Y should be approximately equal to the amount of potential energy gained by moving the ship away from the Black Hole, but from the way subspace is described in the tech room I get the idea that subspace jumps are easier in strong gravity wells, so X could be less than Y. This breaks the law of conservation of energy!
It is far more likely that the energy required by a subspace jump is equal to the difference in potential between the start and end points. Therefore, making a jump towards a planet actually generates energy, since the ship is losing potential energy. Jumping between two points on an equipotential would not take any energy at all, unless friction and other forms of energy waste are a factor.
Besides, where does the gravitational influence of a mass end?
Answer: it doesn't. The Earth's gravity has influence (not much, admittedly) even in galaxies quadrillions of light years away. Hell, even the pen on the desk beside me does!
So if intrasystem subspace jumps are dependant on a gravity well, they should work over the whole universe, because that's the extent of any gravity well!