I wasn't especially intending to debate the merits of unRAID (especially since I've never used it and wasn't recommending it), but what they're doing sounds rather similar to RAID 4, right?
Based solely on their claims (again, I've never used the product), unRAID allows you to assemble a collection of random-size drives, use the largest one for parity, and then give you ~full capacity of the remaining disks. You can expand your volume capacity at any time by adding disks, one or more at a time. It gives you enough redundancy to recover from a single disk failure. unRAID 6 supports btrfs, which does provide checksumming of data and should be comparable to ZFS in protecting from bit-rot--and even if it didn't, nothing else but ZFS does either. I don't see how unRAID's implementation is worse than any other legacy RAID4 or RAID5 solution, and it may be better than some.
In short, unRAID's features seem to fit a pretty significant need/want. We can say all we want that FreeNAS and ZFS are better, but can't we agree that, at least for home use, it'd be awfully nice to be able to safely expand our pools a disk at a time?