The problem is when the two are interlinked, so that altering gamma causes a global change to the calibration - which it shouldn't.
The 601/709 mismatch is pretty small. I've often wondered if the difference between 601 and 709 would be big enough to catch in a simple visual color-bar decoder test.
I wonder what a Rec. 709 vs. Rec. 2020 (wide gamut) mismatch would look like btw? (Pretty substantial I'm guessin.)