I must say I have a few issues with this idea (at least in a non-military application).
The first is the effort of all of this extra development, engineering, production, maintenance etc. etc. for the sake of saving fuel in the smaller parasite craft (of which how many will actually "dock" with this vehicle)? Like public transport, it only makes sense on the most populated routes - otherwise, smaller aircraft will have to detour to use it - it will be the aeronatical equivalent of a passenger car ferry.
In addition, if the craft is going to travel more slowly than the parasite aircraft would ordinarily, it will make journey times a real ballache for passengers. - In this sense it seems that recently we've been making backwards steps. First, we lose ConcordÃ© and with it supersonic passenger aircraft, then, should this come into play, we'll be travelling slower than we are now on 40+ year old aircraft.
Progress? Surely it would make more sense to continue to develop faster & more efficient non-parastic aircraft (compact nuclear reactors couls be an option in the future)?
The only way this would make sense is if it were some sort of stepping stone to spaceflight...
...however, I'm not doubting that an airbourne airport would be an epic thing to see.