I know my missing something, but can't figure out: why not just one device?
Edit: Oops, "zpool create global-pool mirror /dev/nbd0 /dev/nbd1" is a better example for that. If it's not that, I'm not sure what that first example is doing.
I can see it on not real S3 though.
I expect this becomes most interesting with l2arc and cache (zil) devices to hold the working set and hide write latency. Maybe would require tuning or changes to allow 1m writes to use the cache device.