It'd also save the complicated shunting movements at either end and at lower speed running could be more fuel efficient than double 57s.
Are the shunt moves that complicated? AIUI the set works with Class 57s on either end from Reading to London Empty, and then drops off the rear 57 at Reading on it's way back through as 1C50. not sure what the norm is at Penzance - does it propel onto Long Rock or does the 08 come out and grab it?
In the other direction, 1A50 stops at Reading to attach the 57 on the rear and then the empty is hauled back from Paddington to Reading Top/Tail. So there's no shunting at Paddington, minimal shunting at Penzance, and 2 moves at Reading from the Train to the depot and back. The second 57 might even be shut down or reduced to just idling when attached to the rear of the train.
I am slightly surprised that the 'extra' 57 hasn't been replaced by a 90 or similar but I guess keeping 1 or 2 isolated on the GW isn't a great use of resources, as you'd need to get a 66 to trip them across to Wembley Yard if they were needed elsewhere on the electrified network.