You obviously missed the NIrvanix debacle. They simply gave customers a month to get their own data out. Short of taking a hand truck to their data center it wasn't at all easy to migrate everything over the net.

Another advantage of cloud backups is that you don't actually need your computer(s) to be running to perform a backup of a cloud storage account. Local data obviously needs to be booted up, but a Dropbox account can be access directly from the cloud.

Sync is not a backup. What contingency do you have if your provider zeros all your files by performing a faulty sync operation? It's happened. You need to be taking a completely separate copy of that data where the cloud storage provider can't actually touch it.