The part about efficient background sync is a good point, too. I have noticed some minor lagging using curlftpfs in Linux, and that might be something that would make for a better solution in the Linux world, so thanks for that idea.
Your use-case described in #2 does make sense, but I still agree with others' comments here that claiming that it replaces USB drives is a bad idea in general. All of your feedback was well-thought-out and appreciated; I only hope that I was able to give you a sneak preview of some of the potential criticisms you may receive. Best of luck to you!