ZFS still protects you from bitrot when compared to ext4 over mdraid. When you get to many terabytes of data, it's almost guaranteed that you're going to lose something to bitrot. In my case, my most recent scrub detected and repaired 1.58MB of bitrot. And in any given month, `zpool status` will show one or two checksum errors as having been corrected in real-time, as I was working with the corresponding files directly.
This is probably the number one thing that excites people about ZFS over any other solution, and it's something that isn't really easily implemented on a standard RAID + standard filesystem arrangement, since this sort of functionality depends on the filesystem knowing about the underlying disk arrangement.
"ZFS uses its end-to-end checksums to detect and correct silent data corruption. If a disk returns bad data transiently, ZFS will detect it and retry the read. If the disk is part of a mirror or RAID-Z group, ZFS will both detect and correct the error: it will use the checksum to determine which copy is correct, provide good data to the application, and repair the damaged copy."