It helps a bit more than you imply, though: if you can launch from a higher altitude, you have less atmosphere to plow through. That lets you use more of your propellant to speed up instead of to push air out of the way.

You've just got the problem of building a fixed wing aircraft which can carry your rocket full of explosive propellant, successfully release it pointing in the right direction and then get the hell out of the way....

You're extremely limited by the amount of mass you can even launch from a mothership aircraft.

There's no future in this idea outside of small sat, and probably not even there.