Of course it's viable as a backup. Availability != realibility. My data is still reliably saved in the cloud even if there is an outage for a few hours. The key point is backup, e.g. Dropbox. When you use Google Docs, it becomes a single source of truth and a SPOF.
This depends on the circumstances. If your personal photos are unaccessible then maybe it doesn't matter, but if it's your documentation for a mission critical bit of infrastructure then a few hours could be very significant. Somebody in that situation probably wouldn't agree with your assertion that "availability != reliability". If I can't access it when I need it then I wouldn't consider it reliable.
