I think it depends on how Westbury/Frome/Cary are priced. If it is to Paddington, then these services do not scoop the current revenue up. But if it is to "London" then they can collect a proportion of it, even if they end up running as empty stock.
It also puzzles me why so much is made of these services being "run anyway for route retention reasons". How often do SWT ever get diverted via Westbury, and certainly not by Frome (surely if diverted they would use the avoider). But anyway, for driver knowledge route retention you surely don't need to run a whole 3-car dmu, do you? The crew could just ride along in the cab of existing services.
Some of the moves via Westbury also avoid single line conflicts between Salisbury and Sherborne.
I notice that South West Trains have 'warmed up' some of the empty workings into Reading as well.