It's just my opinion, not a right or wrong thing really. Audyssey does its thing in both frequency and time domain. Comparable ones are Dirac Live, Anthem ARC and a few others. I am skeptical about REW based minidsp can do as good a job. There is a minidsp that comes with Dirac Live, that's the one I am considering getting for one of my 2ch system.
Technically and logically speaking, I agree with you that if you EQ it with the basic minidsp first, then Audyssey will have less to do. In practice though, I am not sure if Audyssey would still end up roughly in the same place either way. Besides, the whole point of the Editor app is that it allows the user to "edit" (in other words, sort of manual EQ adjustments) after Audyssey does its auto setup. So with the Editor app now available, why would one need minidsp/REW? Again, just my opinion.