Yes. RAID is a good analogy.
iCloud Photos will protect your photos in case if you lose your device or it fails or whatever. It won't protect your photos if you delete them (either accidentally or intentionally).
It is not a backup, and even "full" device backups WON'T include the photos, because of the (mostly correct) assumption that they are all in the iCloud Photo Stream. They will be backed up in "full" backups ONLY if you don't use iCloud Photos.