Yeah but you can send AZW3 or KFX over USB and convert ePubs to those in numerous ways (e.g. Kindlegen, Calibre). Mobi is old, and is missing loads of features that the other formats support. With the Kindle Personal Document service for a while they only supported mobi but not AZW3, KFX or ePub which necessitated using crappy mobi if you wanted sync across devices. There's really no reason to use mobi anymore unless that's the only format you have it in. And there's pretty much zero loss converting mobi to AZW3 unlike the other way around.