If they had a rocketplane that had a reasonable rocket motor in it that would enable them to do quick turnarounds by simply refueling, doing a little bit of ins

If they had a rocketplane that had a reasonable rocket motor in it that would enable them to do quick turnarounds by simply refueling, doing a little bit of inspection, and reflying, then it might be possible to validate the idea through an extensive test flight program, just as we've done with airplanes. Put in a hundred or so test flights exploring the flight envelope and it would be possible to gain some confidence in the system. With the design that Virgin Galactic / Scaled have now I don't see that as a reasonable possibility, and I don't think it is a safe system (largely due to the rocket, the airframe might be a perfectly fine design).