I think Merc knows pretty well how fast their package is. I would have thought they use the SS and US tyre just to gather some data. Rosberg said that there was no point in testing the US here, because it is already destroyed after S1. But some data should be better than none? Especially if the teams need to pick their tires for the races this season.
Oh well, maybe Merc has another secret tyre test planned later on.