I can understand needing more than 1x73/9 on the Inverness load 8 portion, however it strikes me as slightly odd to feel the need to add a 66 to load 6 outside poor adhesion season and especially 4 which sometimes happens. As
@6Z09 states above, it adds complication to the shunt, plus obviously will use more fuel and tie up more resources. All in all, surely it drives cost into the operation though I do appreciate that traction is GBRf's problem rather than CS. Are the 73s failing to meet expectations (availability, reliability or adhesion?)
Equally (at risk of starting a tirade of people telling me that I am a fool), I would suggest that CS missed a trick when ordering the Mk5s that would have greatly simplified the operation and therefore reduced the operating cost in that the seated coaches could have contained driving cabs to allow push-pull operation. Yes, I know that the gangways are extra wide on a Mk5, but given CS/CAF started with a blank sheet of paper did they really need to be. Coupling could have been banished to Edinburgh and Carstairs only and the number of locos + drivers needed reduced.
Still, a bit too late to change it now.