[zyxley]

> On stock (and jailbroken) Kindles you can send PDFs or MOBIs to [e-mail] and have them auto-downloaded to your Kindle.

That's no good for me because I tend to do a lot of random edits of my epub collection to fix editing errors, replace cover images with better ones, etc. The reason I want Dropbox is to automatically propagate changes like that, not just to transfer files over.

[semi-extrinsic]

Upstream comment recommends running KOreader on jailbroken Kindle for (amongst other things) syncing with Calibre. I'm assuming this is two-way sync, but I don't know.