Actually Picard doesn’t send additional metadata as it sends a recording ID which already identifies the details of the recording.
I don’t know if the AcoustID server would benefit from additional metadata. But as @lukz implemented both the entire AcoustId system and the submission from Picard I assume probably not. It would increase submission size though and mean we need to do more requests to submit the fingerprints.
As I understand it the metadata is useful when submitting fingerprints without MBIDs