I suppose that there may be times in some live playback settings when you might need to grab control of a fade, but have you ever tried to do that on your 300? You have to hit the manual fade button and then take the X1 (or X2) faders to zero and then run the fade, so unless you have a very long count, the chance of you catching the fade is slim. It is even harder to catch a fade into manual on an Obsession. The only time I can think of that you might need this is if something goes terribly wrong on stage.
In the professional
theatre world fade times are set by the designer, and that is that. The show is the designer's vision. a
Stage Manager can only change things if the show is suffering before the change and even then, the SM is still supposed to consult the designer before making a change. A Stage Manager has no "Artistic Control" over the show. The stage manager's job is to keep the show running to the specifications of the director and the designers.