Well, you can control your airspeed and attitude in an airplane, solid rockets just make a ballistic arch and they only accelerate, they can't maintain a speed.

So if you want to test how air behaves in a particular regime, you can't actually maintain that regime with a sounding rocket, so the best you can get is a snapshot.

Honestly, if what they're working on is dialing-in their computer models, the more precise data they can get, the better. It makes sense to maintain a speed for a minute, so you can get a really good running average on your measurements of air pressure, heating, drag, etc. They may want to conduct the same tests at different altitudes and airspeeds, and probably all in level flight. To control all those factors, that's near impossible to do with a rocket.

Again, I don't know what they're doing, what their actual plans are. I'm just saying there are absolutely plausible reasons to use this aircraft.