Hello Scott,
If something bad happens to your primary filer, you can always recover from the replica on a secondary site, but keep in mind that SnapMirror is a "mirror" technology, that means that if you have data corruption or lost data on your source, that corruption or missing data is going to be replicated to your destination; I believe the best approach is to use SnapVault, SnapVault is a backup solution rather than a mirror like SnapMirror, a common strategy is to SnapVault from primary filer to secondary filer onsite, and then SnapMirror your vaulted data to a third filer offsite (DR site).
SnapShots are great for fast, short term recovery (from minutes to days), but they are not very efficient for long term recovery (months +), that's where SnapVault comes into play.
Cheers.