In my opinion it is always a good Idea to use different partitions for different purposes. I always setup one Partition for the OS and another for Data. Often I also use a further Partition for Temporary files, like the swapfile etc and another for Data that doesn't normally change. Such a concept makes things more structured. It also helps for Backing up and restoration.
If your OS has gone bad, it is often necessary to reinstall that OS. If you have that OS on the same Partition as all your data, you will have to restore that after reinstallation. If it is on a separate partition, that data is still there.
It normally doesn't make sense to back up the OS on tape, as it normaly isn't possible to do a restore of the OS without first having an OS installed to access the Backup Software from. So it is just necessary to backup the System State of the OS. So, to plan a backup, it makes sense to backup Data regularly, and if that data resides on a separate partition, you can set up your program to backup that partition on a daily Basis. If you have data, like copies of installation CDs etc that doesn't change regularly and you have that data on a separate partition, then you only need to make backups of that partition when you have made a bigger change.
Another point to keep in mind is that your backup media often only have a certain capacity, so I keep my partitions no larger than the size of my backup media.
Performance issues could arise if you only use 1 partition, particulary from fragmentation and swapping, if you use a separate partition for those activities, it is less likely you will be experiencing fragmentation problems
