Only 26 are needed to run the full rural service. There are 38 of them. Not a waste, the tables and superior seating would be put to good use on the IC.
But it is only 1hr 40mins from London to Norwich. London to Clacton is 1hr 30mins and doesn’t get tables and "superior seating". It is a waste as you are taking out several bi-modes from the normal diagramming to sit idle just in case a 745 isn’t available. For the sake of tables and the seats far better to diagram in some 720's. Then you keep traction knowledge up so more depots sign the traction that is going to be the most prevalent on GEML. It makes so much sense for 720's to form a few London - Norwich services, even if that means some people have to make do without a table for their hour-long journey.