I think this capability would be even better in lower capacity batteries, like the Canon LP-E17 or Sony NP-FW50. When my main camera was a Sony a6400, the OEM Sony battery was 1020 mAh, and I had two 1300 mAh Wasabi spares. I always took all three with me, along with a USB charger in my backpack so I could have the first one recharging from a power bank while I was using the second and third.
Now, with my R7, I don't feel I need this, even though I think it's a great idea. One fully charged LP-E6NH in the camera + one spare is more than enough for a full day of shooting at an air show (and I don't hold back on the shutter button either).