A friend of mine commutes Aviemore to Inverness each day on the Sleeper, if it's on time, and reports that the last three nights were 2x73 but it's now 67013 again. He or I don't know why.
67013 has remained in use over the weekend - however due to the engineering works over the Easter weekend in the Motherwell area it was needed for ECS/Edinburgh portion moves that were running temporarily over a non-electrified route between Polmadie/Glasgow and Edinburgh (these workings have normally been done in recent weeks by 86401 and the F/L 90s when under the wires). I think as these moves are generally 8 coaches plus another loco on the end, the 67 was preferred. It therefore came off its 'normal' Inverness duties for the long weekend to be replaced by the 2x 73s.
Now engineering works are over and routes back to normal, the electrics can work the ECS and Edinburgh portions via Carstairs under the wires and 67013 went back on the northbound EDB-Inverness working in the early hours of this morning (having brought up the Edinburgh portion ECS from Polmadie yesterday evening). I suspect 67013 will now stay on the Inverness (until the next round of engineering...)
73971 has gone back to Brush (again!) - no idea why.
The 67 is an insurance replacement for the damaged 73969 (whilst it is being repaired), it makes sense to put as many miles on that as possible rather than on your own locos.
I'd be interested how the contracts/arrangements work for these - maybe with the 67 as it's an 'insurance replacement' there isn't a per mile charge and it does make sense like you say to use it as much as possible.
However, for the 2x F/L 90s on hire still, over the past few weeks they've appeared to try and use them as little as possible (typically one has worked the EDB portion - the shortest run under the wires, and the other has sat at Willesden; they've only done 'long runs' to bail out 92s on a couple of occasions). I'm guessing in part it's to prove the 92s are able to cope, but I also wonder if on this contract there's a mileage charge to F/L too?
As for 73971 going back to Loughborough, no idea why, however it was working the Aberdeen service the last couple of days and the move to Brush was in as an STP a few days back (i.e. before then), so it's obviously something that's been planned for a bit and isn't something (it would appear) that stops it from working. Maybe something needs an 'adjustment'; or prep work for the new stock?