It may be in how you are visualizing the launch. It is NOT like a heat seeker leaving the rails on an F-16. Although carried underneath, it launches like the original x-planes launched. It doesn't "launch" so much as it is simply "dropped" and then ignited. That way the rocket is safely away from the aircraft before the motor ignites. Being done that way, having the plane pitch up before dropping the rocket doesn't add anything and only complicates the needed flight pattern. Once the rocket ignites, changing course under thrust to point up is easy-peasy.
I agree with it being a more complicated, also agree with
SDramstad, that you don't want to stall the aircraft.
I WAS however thinking that releasing in an upward trajectory provides two benefits.
First, you take advantage of the initial momentum of the rocket, converting it from a purely horizontal vector to a partially vertical one. I'm suspecting that airplane gas is relatively cheap compared to having to build a bigger rocket to hold more propellant. I was thinking firewall the throttles and go to max air safe speed then go to max safe upward pitch, releasing the rocket well before stall speed. Still should have plenty of time to clear the rocket before engine ignition. I have been amazed at how steep many commercial passenger jets climb after take off. It just seems a shame to waste potential vertical momentum if you don't have to. Part of the reason for launching from an aircraft in the first place is to get max efficiency by launching above much of the high drag atmosphere. Every little bit helps. As you said, "changing course under thrust to point up is easy-peasy". It is not however "free", there is an energy cost involved.
Second, using that momentum, you may get a certain amount of vertical altitude coasting before you have to ignite your motor. The higher you get before you ignite your motor, the less dense that atmosphere which is an advantage both in terms of drag and apparently in optimal "nozzle configuration."
On the downside, aircraft may not be able to handle the strain or may stall (both obviously very bad!)
I'm not sure how much propellant you would save, both from converting some of the momentum from horizontal to vertical and from delaying ignition of the motors at a higher altitude. Also not sure how much propellant is spent "re-orienting" the missile from horizontal to vertical (or relatively vertical, eventually the satellite will have to be in a horizontal trajectory for orbit.) My AstroPhysics course was in the early 80s, and I don't remember much, except that energy expenditures for orbital changes are NOT intuitive!
Also if you DO ignite motor higher, your initial rocket fin effectiveness (if you go that route) is less, but in all cases fin effectiveness is eventually zip once you are above most of the atmosphere, so eventually you are using thrusters for changes in attitude anyway.
"Ground launched" versus "aircraft launched" versus "balloon launched"....., major factor is who can find the cheapest AND most reliable technique.
