Well, most backups are for unseeable events. It's your choice to trust DO's contingency plans but I for one would like to put my eggs in multiple baskets, depends of the value of said eggs of course. I wouldn't set-up off-site backups for a personal website for example.
Unless bug in their system, hack attack or human error deletes all droplets from your account. And yes, I have heard of stories like that from other hosting companies where a human error caused the account to be deleted and data removed.