maybe the words backup and raid were a bad choices. Currently (in my setup) if 2 drives or more fail, then I cannot recover the data drive that failed through parity check (as I only have one parity), but the excellent feature of unraid being the other data drives still contain all there data and should be fine to access and use.
So for a share to be mirrored over the other drives making that share for this scenario a much safer bet on the data being safe from this failure.
Also looking at a feature set with something like this you could get improved data transferring out of the server utilising the increased data throughput of parallel access possibly.