Mirroring uses multiple mirrors (plexes) to duplicate the information contained in a volume. In the event of a physical disk failure, the plex on the failed disk becomes unavailable, but the system continues to operate using the unaffected mirrors. Similarly, mirroring two LUNs from two separate controllers lets the system operate if there is a controller failure.
Although a volume can have a single plex, at least two plexes are required to provide redundancy of data. Each of these plexes must contain disk space from different disks to achieve redundancy.
When striping or spanning across a large number of disks, failure of any one of those disks can make the entire plex unusable. Because the likelihood of one out of several disks failing is reasonably high, you should consider mirroring to improve the reliability (and availability) of a striped or spanned volume.