from http://bugs.musicbrainz.org/ticket/3801

Picard tries to map folksonomy tags to the genre field. But this is always going to be a bit of guess so sometimes the results might not be what the user wanted, and there is the additional problem that unless you set the 'Metadata/Folksonomy Tags/Join Multiple Tags With' option some formats such as mp4 will only allow one genre to be saved.

What would be useful if in addition to the genre matching all the folksonomy tags were saved to another field as a comma seperated list. This should be a new field currently not used by anything else, i.e for Mp3s it could be stored as TXXX:MusicBrainz Folksonomy. Then users could have full control over what they did with these and use them as required, e.g none genre purposes.