Assuming you're getting quality backups (that you've tested, of course..
) there is no reason for a separate data partition.
If the backup file(s) or image(s) become corrupt...or if you have a physical drive/mechanical failure, then you'd be at a loss.
I keep my data on a second HDD and once every month or so backup what's important to me (mainly pictures from over the years) to a few DVDs.
Just good practice.