I kinda like 21700 too but for different reasons. I have/had a 26650
mod which also had a larger than 18650 battery but it had charging issues. Because 2 1860s charged effectively twice as fast in my charger than 1 26650 it would only be partially full so I couldn’t just swap two
batteries. I needed 3. With a 21700 99% of the time I can get away with 2. I don’t know if the 21700s have a higher charge rat than the 26650s or what, but with them it usually works. I bought 3 when I bought a 21700
mod, because of my experience with the 26650, but while I do find it handy, the “spare” will sit in the charger (doesn’t hurt it. I bought an expensive charger) for weeks at a time. I could have gotten away with 2 21700s and then use the included adaptor for the few times I stretched it too far. I’ve had at least 3 two battery 18650s and I don’t know how many singles, so I’ve got a bajillion of the things. Most are mostly dead but would work for that.