"To me, this seems to be a somewhat questionable argument, since there's no way to know exactly what color an object should be in the expanded gamut after it has been reduced to Rec.709."
This is a very good point and one that popped to mind immediately when I saw the "DCI"gamut "feature". Personally, I'd calibrate the set to conform to REC 709 and forget the extra colors. Mapping REC 709 colors back to DCI ranges is what we in the Computer Science world call "indeterminate" and is generally a bad thing. If they could give you an accurate LUT to plug into some color box, then I'd be interested.
Exactly. If your calibrating to REC 709 the wide gamut is only a hindrance.