Re: Why air launch?
Launch when you want without worrying about rain and wind at the launch site.
Yep, definitely a plus. Flexible launch sites are also part of the attraction of sea launching.
It's all about the size of your nozzles
Avoiding sea level-optimized nozzles helps a bit, but it's not everything. For example, the shuttle's SSME nozzles were a compromise to operate from sea level to vacuum but only gave up a bit of vacuum specific impulse. They eventually having achieved 453 seconds in vacuum (where they spent about 6 of their 8 minutes operating), while a purely vacuum-optimized nozzle would've delivered 455 seconds.
For multi-stage, non-parallel rockets, the rocket nozzle advantages of air launch are even smaller. The Pegasus gives a good data point...
For a once off launch, ground launch is clearly cheaper, but at least 98% of the take off mass is lost. (Saturn V took off weighing 3,000 tons to get 40 tons on the moon and 10 tons back to earth)
...Understood, but subsonic air launch isn't giving you much in launch mass savings. The 18,500kg Pegasus has a 443kg LEO payload (40:1), which is a worse payload ratio than the ground launched Atlas V (18:1), Delta IV (26:1), and Falcon 9 FT (24:1).
The point of that comparison is that air launch mass savings are small enough to be wiped out by differences in propellant selections. Despite being carried to 12,000 meters and 270m/s by an L-1011 carrier aircraft, the solid-fueled Pegasus has lower mass-normalized performance than liquid-fueled ground launched vehicles.
The all-solid, 4-stage Pegasus II slated for the Stratolauncher will also fall short of liquid fueled, ground launched rockets in performance.
For repeated launches, once the cost of the bits lost getting to the air launched altitude match the cost of air launching
You don't save much with an air launch except fuel, and rocket fuel is one of the lower costs in rocket operation. The purchase price of a $100 million used carrier aircraft like an L-1011 would fuel a lot of Falcon 9s.
Biting the hand that feeds IT
