I agree that for traditional launch systems it doesn't make that much sense -- particularly in light of the recent development of reusable (or at least, partially reusable) rockets.

That being said, I think the math changes somewhat for smaller launchers. The smaller the rocket, the worse the losses to the atmosphere get, and the more advantageous it is to escape atmospheric resistance in the soupy low atmosphere. So I think where air-launch systems could really shine, is by enabling low-cost launch of very small payloads.

I am, however, not entirely convinced that a sufficient market for those kinds of dedicated launch services will emerge (as opposed to hitching a ride in spare fairing space of a separate launch). I could see this going either way; as much as I'd like to see easier space access for eg. university-funded nanosats, I'm not sure the timing will be right for something like Stratolaunch, because the new private aerospace (SpaceX, Blue Origin, etc) is focused so intensely on pushing down the cost of larger-scale rocketry, to a point where that same market would be better served by bulk launches using some kind of standardized dispenser.