One of the coolest things that git-annex lets you do is define remote repositories on services like Box.com and Amazon glacier for backup or to use as a transfer between nodes.
I know Syncthing expects all nodes to be full Syncthing clients so this isn’t immediately reproducible. But do you have any intention to integrate cloud storage of one kind or another in the future?
I don’t myself use any cloud storage (in fact, syncthing is a reaction against not wanting to use cloud storage), so no, not for myself. But it’s certainly a valid use case and I wouldn’t object if someone wanted to contribute the functionality.