There was a discussion on HN about this last year, but with Dropbox being the issue: https://news.ycombinator.com/item?id=4703943

Dropbox, Google Drive, Skydrive are more of folder mirroring, much like RAID is data mirroring. Any good sysadmin will remind you that RAID provides protection against hardware failures, but in no way is data backup or protection. Onsite and Offsite backups are always important.