One of the reasons I'm thinking of having two partitions, is to have one (the media one) be readable (perhaps (parts of it) writeable?) for all (or selected) users on the system, while the backup would only be accessible for my own user (or perhaps only root). The backup would also not need to mounted all the time and could be tucked away at the end of the disk and shrunk/expanded as needed, while the media one could slowly be expanded upwards towards taking up most of the disk. This would make the seeking for media files not being cluttered by having to mingle with backup files, and vice versa, and would keep the backup files less prone to being corrupted, simply by not being accessible as often as the media files.
And XFS doesn't support file system shrinking. :/