About the only use is as part of a rolling upgrade, so install a second SQL instance (same version), configure mirroring, make sure everything works and the app redirects properly, upgrade the mirror (now running higher version mirror than principal), fail the mirroring over, upgrade the principal, fail the mirroring back (or, if you're migrating hardware as well, leave the 'mirror' as the new principal).
Version upgrade with minimal downtime (one or two failovers).
In a rolling upgrade though you wouldn't install the mirroring with mismatched versions, it would just be that way for a brief time during the upgrade.
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