From the Apple Website
It automatically backs up everything, so you never have to worry about losing your important files.
Is this persons claim silly, yes. Does he have an actual case? Yes.
No where in Apples packages or brochures does it ever say "This does not guarantee that your data will remain save, make sure that you have more then one of these". All that Apple ever does is talk up their product and not worry about the rest. For people on the forum here, this seems logical "well yeah, backup shit to more places". But every day I deal with at least 10 people who don't even know they should have 1 backup, much less more then 1. Whenever I purchased an external HD that came with backup software, the very first thing that I read was a warning that told me that backups are not guaranteed and you should make sure you have more then 1 backup of important files.
As a tech person, you kick yourself when shit like this happened. "Ahhh, fuck. I knew I should have saved that file to this other hd to". When you don't know anything about computers, you sue the person who promised your data would be safe.
I feel Mac users who get viruses should also sue Apple. Even if they don't get away with it, it will teach Apple that they can't just say whatever they want, and it will raise awareness for people who Apple products that they are not virus immune.