This has a few advantages that are not apparent immediately: 1) Rocket engines work a lot better at low ambient pressure. You get more thrust for the same fuel

This has a few advantages that are not apparent immediately: 1) Rocket engines work a lot better at low ambient pressure. You get more thrust for the same fuel flow. You can fly with a bigger nozzle, increasing this benefit. This is probably bigger than the altitude and speed benefits in subsonic airlaunch. 2) Air launch is flexible since you can fly a long distance to a secluded spot over the ocean and can launch in any direction. You could fly polar flights from Florida. Or low inclination orbits from California. Both of which are impossible for fixed launchers. You can fly towards an orbit's ground track to get flexibility in launch time as well. 3) If you have a problem with the early rocket flight stage, you have enough altitude that gives you more emergency options. For a ground takeoff rocket, the early part of the flight is very dangerous. Reaching orbit is really hard. It is extremely hard with a single stage vehicle. The problem is exponential in nature. The required mass ratio is proportional to e^(required delta vee / exhaust velocity) For a reference design, with 3500 m/s exhaust velocity and mission delta vee of 10 km/s, you'd need a mass ratio of 17. That means the vehicle is 16 parts fuel and 1 part rocket and payload. Helping a single stage with an aerotow to 0.3 bar ambient pressure, 10 km altitude and 300 m/s speed might just help it enough. Say if delta vee is reduced 10% and average exhaust 10% as well, the required mass ratio suddenly drops to 10! To demonstrate what that means: if you could make the ground launched reference rocket plane to fly to orbit with a 3,5 ton empty weight and 0,6 ton payload, (70 ton wet mass), the aerotow version could replace 2,9 tons of propellant with payload, meaning 6 times the useful payload. These calculations are not very accurate, and are very sensitive to the reference design's payload but they give you an idea of the nonlinearity.