This applies to related subtypes (mixed/recorded/...) too.

I come across a LOT of "vocals recorded here by person X" and "strings and horns produced and mixed by person Y".
Currently these require annotations to preserve the info (I also tend to use the "additional" flag on the ARs where possible). It would be really nice if these could be added (whether as attributes for the main AR or via specific subtypes of the relationships, the way it is for "performed").

With 3-way relationships that would be mapped even better of course

Examples: http://musicbrainz.org/release/a84ca3d4-fbfe-4c84-8d89-92082d51990e (also has a case of [instrument] conducted by) and http://musicbrainz.org/recording/b3541d85-f9f5-48f6-9044-1441b93e83b5