IIRC, there are also some Kindle-specific concerns with the busybox tar version they ship.
Plus, I'd rather not touch the checkpoints code, it was already annoying enough to get right, and has ties to the OTA manager, which we really want to make sure behaves consistently across all supported platforms.
Also, compared to DIY approaches, it has the advantage of being actually linear, as it's based on data length, and not file amounts
.