Often when Mingus would sing like this it was 'cuz the band was lost or wasn't phrasing something the way he wanted it -- he was modeling what he wanted them to do. Since he was busy playing the damn bass, he was unable to go up to the players to punch each guy individually in the face; hence the singing (perhaps amplified here in a way Mingus likely didn't intend, thanks to the announcement mike?).
Gilmore's lovely here. Mingus is lovely here. Here's where this came from:
[EDIT: by the way, this is not, alas, a film of the entire performance. (In fact, performances regularly start cooking right when the film idiotically cuts away.) Instead, it's an intense, amazing, frustrating, and sometimes appalling document capturing some moments and aspects of the intense, amazing, frustrating, and sometimes appalling Charles Mingus.]