This seems to be a very nasty bug to me. Anyone installing backupc and expecting a no hassle backup of 'localhost' will be in for a shock, as the default tar arguments simply won't work under the default 'backuppc' user. There will always be at least a few files that don't give world-read access. What's worse is that backuppc will just keep trying to do its first full backup, trashing performance on that pc.
Including --ignore-failed-read as a default argument to tar seems to be a simple and effective fix. If users do want a full backup including protected files and take steps to do so e.g. run as 'sudo', it will have no effect.