I think that combining the administrator permissions to create backups and to restore them is problematic. I would like to give a trusted administrator the permission to create backups of the database, but I don't want that user to be able to restore backups, maybe even accidentally:
http://tracker.phpbb.com/browse/PHPBB3-11349
http://tracker.phpbb.com/browse/PHPBB3-10239

It's not just a matter of trusting the administrator not to abuse privileges - there are some completely unneeded privileges which I don't want to give because their only use would be destructive in my case.

Furthermore, there is a quite big number of users which I would trust enough to allow them to create backups of the database... as long as they can't download them. It would be nice to allow any moderator to create a server-stored backup of the database, and it might prevent data loss if the most recent backup needs to be restored.