I think Thomas talk about (FAQ MetroCluster - June 2012):
Can non-mirrored aggregates be used on MetroClusters?
Yes, non-mirrored aggregates can be used for both Stretch and Fabric MetroCuster. However, with Fabric MetroCluster all
storage must connect through the switches (no direct attached), and with Stretch MetroCluster all storage must be seen by both controllers.
Also, non-mirrored aggregates will not be available during takeover and multi-disk failures on non-mirrorred aggregates
can cause the system to panic, thus affecting overall availability of the MetroCluster.
For my opinion, we can take the risk.