speaking for me only - that would be one more thing to carry (well two - the battery REI hand warmer and then something to put the battery/hand warmer into - perhaps a very small Pelican case?) - - so for me, more to carry...more to spend...when I can just put the battery in my coat pocket and achieve the same end...save money and less stuff. I just have to be less impulsive and remember I am tethered...like a spaceman. I just cannot wander off to the dark side of the moon while space walking. And besides, I don't have any 1.0 lenses.
In the case of the Nikon Z series, someone seems to have made a battery that will fit into the flash attachment area of the camera. The Z seems to require a smaller (less powerful) battery (10,000 mAh)than my Canon 5Ds (20,000 mAh)...so different approaches are possible with the Nikon than the larger Canon.