henners, the additional track time for the drivers alone is already a big advantage. We're still early in the season having 3 additional GP distances (or 1.5 each) to further get comfortable in the car is already worth a lot, even if the engineers couldn't get a byte of data. And even in unrestricted testing a lot depends on driver feedback. Not everything can be determined by sensors.
Also, I didn't read anywhere that some FIA officials were there to supervise it. Do we take Mercedes' and Pirelli's word for it that they really didn't gather any data?