well it depends on your setup.
If I read Tim's post correctly, BackupPC doesn't offer a FULL backup or Bare metal restore.
So if you have a hardware failure, then what do you do?
I would use BackupPC to backup file level data, in case users delete a file by mistake etc.
Then I would use Mondo as a FULL backup restore in the case of a Hack, hardware failure, or whatever.
Then to provide the client with a extra level, if you add SDN remote backup tool, which would allow you a off-site backup in case of fire, death or plague! hahaha
P.S. My understand Mondo is a Image backup tool, doesn't provide file level backup. So you can't restore a specific file if you need too.
It would be nice if the tools were connected, as managing them both would be a pain. However, both work with cron (or as I like to call it in the forms... con job)... so you can automate in some ways.