I thought the purpose of Mass was Adoration, Thanksgiving, Atonement, Petition but apparently one of the New Liturgical Movement blogggers thinks there's a fifth purpose: boosting the musician's ego. There was a post arguing that it is mean and in bad taste to leave the Church while the organist is still playing the after Mass music. Nuts to that!
When the priest says "Go! The Mass is finished," and he walks out you can leave. I stay to say my thanksgiving prayers, Rocky usually talks to Father Theoden or Father Frodo or he helps carry something into the sacristy but we aren't hanging around to hear the music.
Our organist is actually a sensible man. He plays just enough to get Father out the door and then he quits. If your organist is playing 5 or more verses after Mass and Fatehr has long gone there is no reason in the world indulge him. It may be bad taste to leave but Holy Mother Church doesn't say we have to stay until the organist is done. Gee whiz.