> it'll make sense to just designate the last 10% of battery as "reserve" and not count it in the battery level indicator.
Very questionable? Why would you do that?
We don't do that for gas cars either.
I think the use cases for field-swapping a (part of the) battery pack are the same as for carrying extra fuel canisters with you, which I can only speculate about, because I've never been in such situation with an ICE car.
I once, embarrassingly, found myself on the highway with an empty tank of gas and 20 miles to the next gas station. I watched the estimated miles remaining indicator tick down mile after mile, ticking precisely my passage. At 3 miles estimated remaining, I pulled over because there was a very wide safe shoulder, and I didn't want to putter out in a less-safe spot.
I don't want to try the experiment of letting it tick down to zero and seeing if I still have 10 miles or so left.