...on the stature and competence of the joiner and joinee. If the artist brings on a personal friend from back in the day and the audience has never heard of him, it's usually a debacle or an unmusical distraction. Van for a while had a habit of turning over half his set to whoever he was favouring. It was almost fine and dandy for the audience if it was Bobby Bland, but it was godawful when he wheeled out Chris Farlowe.

I remember the energy level, already at fever pitch, when Pete Townshend joined Springsteen at the NEC in 1981 to add a few windmills to Born To Run. Quite exciting, if not musically. Jackson Browne playing piano with The Eagles at Wembley - nice to say "I was there" but again hardly a musical memory.

A classic of unorganised chaos - Zeppelin with Keith Moon, 1977 - (comes in at about 5.35):

I vaguely remember Nick Lowe joining Johnny Cash onstage at the Albert Hall for a spot of merriment. That worked pretty well but Ron Wood guesting with Dylan on Like A Rolling Stone at the Hammersmith Odeon in the early 90s (Mark Ellen sat just in front of me) was a disaster.
This however is pretty good. The Felice Brothers joining Bright Eyes for a version of Tom Petty's Walls.

Joined by Edwin Starr for a rendition of War at Villa Park in the late 80's. Fun and all that, but as I remember it, it seemed to develop into an I-can-shout-the-word-WAR-louder-than-you competition.