http://musicbrainz.org/doc/Style/Unknown_and_untitled/Special_purpose_artist says:

"[dialogue] - Used for soundtrack tracks which consist solely of excerpted dialogue. Example: the "Royale with Cheese" discussion from Pulp Fiction would be correctly assigned to [dialogue], not to "John Travolta & Samuel L. Jackson". Their performances within the track should instead be indicated by vocal relationships."

That seems a bit strange in combination with the artist credit concept of crediting who's credited. We should dig into the history to see if the rationale was just to avoid creating collaboration artists, and probably consider modifying it to apply only to uncredited dialogue.