I still say Dropbox provides a legitimate backup, consider the features my free Dropbox account offers:

  (1) Redundancy - All files in your Dropbox are saved on every computer and stored on Dropbox's servers
  (2) Accessibility - Files in the Dropbox can be reached from any computer with internet access via a browser and downloaded
  (3) Versioned Storage - Every saved version of every file in the Dropbox going back thirty (30) days is available online
  (4) Deletion Protection - Every file in the Dropbox is available online and can be restored, even after the file is deleted from the computer and removed from the dropbox

I may be missing something, but what exactly does a "true" backup system do that Dropbox doesn't?