It can be a backup client, as long it's not the only one. I use OneDrive + a local NAS as my backup. Both have advantages and disadvantages, but having at least one copy locally and one copy in the cloud is quite safe. Obviously these two must not be synced in realtime (because if the sync software screws something up, then you might lose everything) -- so I just keep everything on OneDrive, and every couple of weeks manually copy the files (this is still not 100% safe because of possible file corruption issues, but it's OK for me).
