A set of 5 stripes of mirrors has the same problem that the comment brought up - losing the _wrong_ two disks loses all of your data.

In this case, going to raidz2 across all 12 disks would give an extra disk worth of capacity/performance with the ability to lose any 2 disks without data loss and going to raidz3 should give the same capacity/performance with the ability to lose any 3 disks without data loss.