The only reason I can think of is that they'd like to be able to use Shotts for diversions if necessary. LNER don't need to run services through Gainsborough and Hexham on a weekly basis for that.
They use the Shotts line daily at least once.
5C49 shuttles a unit back from Central daily, as and when the inbound service actually makes it to Central and isn't capped early at Waverley.
There are also one outbound diagram from Craigentinny in the morning, and a second return at night.* Crosscounty runs one more service terminating at Central daily than departing there, and 5C49 gets the spare unit back to Craigentinny.
* There has however been an effort on XC's part recently to stable the unit at Polmadie instead, and send the driver back on pass, and therefore it's more often than not that either or both of these other workings run instead to Polmadie.
It's very likely therefore that there would be a high proportion of XC drivers who would already sign Shotts, and definitely doesn't begin to explain why they want to send a unit from Craigentinny on a three hour jaunt to Central via Shotts and back, and back to the depot, every Sunday from the middle of December. And it definitely doesn't explain why they want the Government to subside it....