I think the idea is that if it is possible / easy to re-obtain the media, then you don't need to back it up (i.e., if it was obtained via bittorrent).

A second classification is data that isn't changing -- you can create a couple backups of that media, store one in a bank safe deposit box, maybe ship another backup to a family member across the country.

Then you have recent acquisitions and ongoing projects -- those are what you would sync daily to an online backup service. So you still have offsite backup of everything, but by splitting up the categories, you end up not needing to sync that much online.