The same can't happen with Dropbox, because minecraft worlds are NOT stored in ~/Dropbox/ folder. That actually reinforces my point about a separate synced folder being a plus for most users, not a minus.
They certainly can be. They can be stored wherever you like, either if you are using a modding/hacking launcher (both are very popular for non-technical players, e.g. kids who aren't programmers) or if you are running a minecraft server (the vanilla server jar runs wherever you decide to put it, and doing this is not exactly an esoteric feat; again, nontechnical kids do it.)
So I myself simply sync just one folder with Syncthing, Dropbox style.