Because I don't want just Linux. I use Linux, Android and Windows regularly.
Also what I'm after isn't offsite backups but seamless directory syncing (with cloud storage to eliminate the need for peers to be online simultanously), which is related but not exactly the same.
Dropbox, SkyDrive, Google Drive fit this niche generally speaking but Dropbox is the only one that has full coverage across the OSes I use regularly.
SpiderOak does zero knowledge backup & sync across Windows, OSX, and various flavours of Linux, and has Android & iOS clients; 2GB free + 1 GB for each friend you refer, or $100/year for 100GB (deduplicated) and no limit on devices.
The UX is not as nice as Dropbox, but I've been using a free account for about 2 years now and I'm pretty happy with the service and the support (especially given that I'm not a paying customer).
Quirks are that deleted files count towards your quota until you delete them from the list of deleted files (deduplication makes this a bit less painful), and there isn't a way to bulk restore files deleted after a certain time.
The abstraction the whole idea. Dropbox is expensive capacity that has much more functionality than offsite backup. It's multiplatform online shared storage with good disconnected operation support, file history, dedup, etc etc. Different use case.
They have only Windows and OS X clients, officially[1], for desktops (Android and iOS are also both supported). They do have a public API, so you are free to make whatever you need to if you wish[2].
