AcoustID has 45M unique ids, MusicBrainz has 20M so there are roughly 25M recordings in AcoustID that can not be matched to MusicBrainz, and this disparity looks set to increase over time.
So for these songs the metadata submitted when a fingerprint is added to AcoustId becomes more important. Although this data has to be used carefully as there is no 3rd party verification it can be used to provide some basic identification of songs.
AcoustId currently returns:
if they were submitted with the fingerprint
But could this be extended with two common fields that would be found in alot of user submitted metadata, trackno and albumArtist.
If AcoustId returned TrackNo then we could construct basic album metadata for a group of songs that could only be matched to AcoustId.
If AcoustId returned AlbumArtist, this would be very helpful for identifying if the album is a various artists compilation release.