The only use I know of that is made of Stormstown sidings these days are to stable Pontypridd terminating services out of the way until they're ready to return south.
Edit: Here's one example for today on realtimetrains:
2J12 Barry Island - Pontypridd continues empty as
5Y53 Pontypridd - Abercynon with an hour's stopover in Stormstown, then forms
2Y53 Abercynon - Barry Island
An evening service does the same routine shortly after 6pm.