Trigger cable lowers temperatures & power consumption

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Speculating a little, but not too wildly:

• It's likely that the processor(s) on the companion could go into a very low power state from which it could be woken on receipt of an interrupt; I doubt the S/P-DIF inputs (whether on RCA or XLR connectors) can generate interrupts on a transition, but it would be trivial to do this using the trigger input which I guess is just a discrete digital signal (aka GPIO).  In other words, the master just has to send a high-to-low or low-to-high transition via the trigger cable to wake up the companion from its deep sleep.
  
• The configurator might not support the trigger cable when the master/companion connection is via RCA-XLR because that configuration is inherently asymmetrical, as the XLR connector is an input only.  This means that the companion can't transmit data from any of its inputs to the master so the companion inputs can't be used.  That would make the trigger cable redundant if you consider its primary purpose to be related to input switching (rather than waking up the companion).