The main reason for me to make different partitions on my RAID arrays is of course, that I hate 10 - 20 TB monster partitions. Also the smaller the cluster size is, the bigger file allocation table (FAT) will be. The bigger the FAT is, the slower the operation system works with the disk. In boot partition I use a small cluster size, in media partition maximal. I also need to have at least on FAT 32 partition for VM testing purposes.
I agree that just for backup purposes a separate partition is needed only if you have only one hard disk and no other storage for backups.