Do you really gain much from a carrier plane if you want to reach orbital velocity? LEO velocity being ~18,000mph, and any realistic carrier plane going much slower than that.
Can't save that much on the orbital velocities. The main takeaway is the capability to launch on-demand, from different locations. In which case it suddenly becomes a logistical challenge : i.e how do i prep the system to launch X rockets with short turnaround.
Bear in mind that the payload is limited by what the carrier aircraft can hold. Either use an existing plane and retrofit like what Orbital does , or build an entirely new plane.
There's a bunch of heavily funded startups trying to launch small payloads into orbit, which is always interesting to watch.
There are some pretty big advantages, but speed isn't a big part of it.