Interesting topic indeed! What I think is a major missconception of MIR is that you just put your Intruments in the room where you think they belong and except everything to sound great. To be honest: when MIR was anounced I also had high hopes that I wouldn't hve to care for mixing that much any more in the future. I guess it will always take experience and skill to do a great mix, no matter which tools you might have ...
In my opinion, the problem is that most people use samples for a filmmusic kind of sound (even if it might not always be filmmusic they compose). When you use the Vienna Konzerthaus main stage it sounds more like a classical recording. What DG wrote is exactly, what they do on a filmmusic recording most of the time: They record on a rather small stage (compared to a concert stage) and do plenty of manipulation, especially reverb, in the mix.
I am curious, what MIR pro will do in that regard. I hope it to be more a "creative stagesound manipulation tool", where you can tweak each instrument the way you whant it to sound like. E.g., it may be realistic in theory, that every instrument sits in the same recording room, but it is not necessarily the best result for the sound you are after. Maybe you want the brass to be in the konzerthaus, but for the Strings you prefer a small stage with additional reverb to compensate, etc. ...