Do you treat these systems as "reliable" and "safe" backends (since they're physically and geographically replicated) or do you feel it's still necessary to back up data hosted on their services?
How/where do you back up that data to, especially with large amounts of data (e.g. 100s of GB+)?
S3's entire business revolves around never losing anybody's stuff. They have hordes of smart people working on the problem, and they have an architecture that makes it really hard to lose anything by accident.
My business, on the other hand, revolves around letting people draw cartoon testicles onto other people's powerpoint presentations in the pretense of a "web meeting". Which of us would you rather trust to keep hold of your valuable data?