Then it needs to be justified on these criteria, not on the basis that it might be used on Summer Saturdays. If you can justify it on these grounds then yes, it makes sense to use it 15-20 extra times a year. It would be daft not to in fact if you could be sure of filling the trains. However even justifying expensive stock to stand idle most of the day is pretty tricky, never mind for most of the year.
This is where privatisation really has helped to kill off these trains. BR used rakes of (fully depreciated) mk Is to Aberystwyth on summer Saturdays (that during the week worked peak time workings to Northampton), hauled from Shrewsbury by class 37s off Canton that were idle at the weekend. The Oxley-based mk II aircon sets that normally worked between Shrewsbury and Euston in these paths could then work on other summer only trains to the West Country, North Wales or wherever. Essentially BR could do what it liked with its stock without all sorts of complicated contractual nonsense. How could Cross Country (for example) get its hands easily on London Midland sets without a lot of hassle and more expense?
Look, I'm not against the operation of summer extras, just understand why it isn't viable.