Here's an inside look at what we've been working on...
--redirectcommandline argument, allowing to specify a filename to redirect SpiderOak commandline output to a file. Especially useful on Windows.
--datadircommandline argument, allowing to specify an alternate path to SpiderOak's data directory
You could sync two folders on the same machine -- perhaps one is a a network drive folder, or a folder on an external hard drive that is only sometimes connected.
You can sync your Documents between work and home.
Sync your pictures between home desktop and and your laptop.
You can sync your music folder to your music player (if it works like a USB drive -- iPods don't.).
You could sync your Desktop folder across Ubuntu, Windows, and Mac.
You could sync your local website folder on your desktop to the website folder on your Linux server that serves your actual website, so changes go live after you make them.