Yes the air-launch system does reduce the weight of the launcher.
The Spacex launcher takes 1:10 to get to 10km (30,000 ft), at a speed of 1400 kph. This from a first stage launch terminating at 2:40, at 66 km and 8,400 kph. So without getting bogged down in complex calculations, the air launch system is saving about half the fuel load and weight of the first stage booster.
And while that is a saving, I am with others here on thinking those two huge empty fuselages are a waste of time and weight. A couple of torpedo fuselages would do, with the wing raised above on a pylon, T21-style. (This thing does not need the super efficiency of an airliner, it is only circling the field). And join up the tail surfaces too, for a much stronger rigidity-box. Oh, and why the great length of those (empty) forward fuselages? You could chop the forward section in half, perhaps adding some ballast to compensate. And having done so, you could then reduce the size of the tail surfaces - saving tonnes of weight and making the thing look more elegant. The old saying is 'if it looks right', and this just does not look right.
But the big problem of this air launch system, is you get stuck at a certain maximum payload capacity. With SpaceX, Musk can just strap on multiple boosters, and double or treble the payload. (Athough a formation booster re-land would be interesting to watch...)
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The Slingsby T21 wing pylon.
