Apart from all the potential local passenger traffic (not all going to/from Waverley/Haymarket) there would be opportunities for trains from outside Edinburgh to go to Galashiels without going via Waverley - change at Edinburgh Park if people wanted to go to centre of town.
Imagine getting a train at Bathgate changing at EP getting on a train that might have started at Dunblane, and seeing people getting off at Newcraighall (for QMU) with a shorter journey time than if they had changed at Waverley.
Yeah, they could do that now if they wanted to. Just the renovation of a few stations to pay for, and some rolling stock maybe. Rejig the timetables and you're off...
IIRC I read something in the early 1990s (when I lived in Morningside) that the South Sub was used to transport nuclear waste. I presume going from Torness to Sellafield for reprocessing?
Indeed there is a loop from the East Coast mainline which connects to the South Sub via Newcraighall. Passes through the goods yard at Millerhill/Whitehill.