You do not really gain that much velocity from a airplane launch.
LEO requires about 17,000 mph. At best, that airplane is going 500 mph when it releases the rocket. Now you can also take advantage of the earth's rotation and prevailing winds to get higher speed, but you only get 5-6% of the required velocity.
Altitude of a LEO orbit is a minimum of 100 miles. IF the airplane is at 50,000 ft altitude (very generous assumption) that is less than 10 miles altitude.
The downsides are that the rocket is expendable, and more importantly, the rocket has to be able to take the side forces of takeoff, turbulence and potentially landing ( I think they have to be able to scrub a mission after take off and return to the hanger without launching if it is going to be commercially viable). This means greater strength and therefore greater mass for the rocket, partially negating the velocity and altitude advantages.
The military has been using the Pegasus launch system for quite a while; that's an air-launch system for getting into low-Earth orbit.
Also there's an ASAT weapon system that launches from an F-15. The concept seems to be cost-effective under some conditions.
Yes, but the rocket is heaviest at launch, so even though 10 miles is only a fraction of the way up, one has to burn an awful lot of oxidizer in a rocket engine to get there. By using an air-breathing engine, this oxidizer, as well as the extra fuel (and oxidizer) needed to lift and accelerate it, can be dispensed with.
Yes but the atmosphere is a lot less dense at 10 miles up, less geat, less friction, less drag, less fuel, less weight going up. To start your rocket flight there probably has great advantage over on the ground.