Do you think that there is a need for some kind of ECC at the OS level? There are surely some applications where even 1 bit flip in 25 petabytes is bad. Amazon's 2008 site outage comes to mind: http://status.aws.amazon.com/s3-20080720.html
Also, EC at the block level would probably spread the blocks around the drive. This means any read would need to seek all over the dang place trying to reassemble files and that's a bad access pattern for spinning disks. Real bad. Like, the worst. It might even reduce the effective lifetime of the drive. So it would be not only correlated with device failure, it could precipitate it.
Maybe it would be ok on an SSD.