You are correct there is a path in the engine that could have been used for communication between the two directors.  However that would have involved two different code paths for the software to deal with.  Also there are hardware functions included in the chipset for the fabric that are not present in the interconnects built into the engine.  The RIO fabric is very low latency and is not a bottleneck as compared to the internal communication path.
Folks,       I am trying to understand how the virtual matrix will be utilized in SE.       My understanding is that directors housed within a single engine will communicate using the cross-director link. Virtual matrix is only used for communication between the directors that are in different engines.       If this assumption is indeed correct, I cannot begin to understand when and why a virtual matrix is required in SE.       Anyone ?  - S 
MIBE and VMAX SE