[Sabardeyn]

I'm not sure myself. Until recently there was nothing for me to sync since I mostly used calibre's viewer. If I had it on my smartphone, I would delete the ebook once I finished reading it. Even now I doubt I will keep books on my device for an extended period of time. Most of my reading is entertainment so there is little need for long term storage. I am using Calibre Companion to transfer my books which should make updating any books easier - if necessary.

Edit: This is a cross-post with theducks' message, above.