Rationale for swap on microSD is IMO avoiding I/O conflicts with other eMMC requests. I'm using it for months now, and for me, it's a must-have. Yea, You may not notice difference in day-to-day use, but under high I/O + high swap usage, You'll feel it Sometimes, to the point of crash (without swap on microSD), as Easy Debian thread prove.

As for battery usage, I don't see any real life higher usage. In terms of both subjective feeling, and "scientific" measurements (average current tracking). I think mounted SD card is somehow "always in use" in terms of interfaces power usage etc. It's also possible that I'm wrong here, but impact on battery life is close to 0 anyway