I think the assumed issues here are the long rebuild times and the expectation of an additional failure happening during the rebuild which could then trigger another disk error. Then the entire array of data would be taken offline and possibly corrupted. I have build many RAID5/6 arrays and have rarely lost any data, but I am leery and now and tend to just stick with smaller RAID 1 or 10's due to the large size of current disks. We really need native ZFS (BTRFS?) on everything now. Data should be automatically distributed to multiple disks and the file system should be able to guarantee via checksum the data read is what I wrote.