Kiwidude, this is really just a question, not necessarily a suggestion...
Is it possible maybe down the road that there would be some mechanism for doing the opposite - marking books for removal
from the device on the next sync?
Reason: When I finish a book, everything from that point is usually done on the computer - update Goodreads, mark as "read" in calibre, then browse my TBR lists in calibre looking for the next book. I (for some reason) don't like deleting books from the Kindle, and by the time I hook it up again, I often forget to do so via calibre.
It's entirely possible that I'm the only user who would use something like this, but I'm sure if I'm wrong, my twin Nyn will be a long momentarily to correct that notion.