Well it's just a bonus on top, I'm not counting on it. Storing solar and backup is the primary reason for the battery. On June 24 at 19:00, agile octopus prices were about £0.85/kWh, there isn't a single simple price for electricity.
It's true that there isn't a "single simple price" but the only way you're getting more for your power than a wholesaler can sell it for is that they're paying far more than it's worth. Stability services aren't free, but we're not talking about a 10% premium here, closer to 10x