I believe one song where a guest appearance makes the song is when Steve Wilson of Porcupine Tree takes over on the song "ShutDOWN" on O.S.I.'s first album.? After hearing Kevin Moore's voice for the whole album up to that point, Steve Wilson just comes out of nowhere and wakes you up.
An example of where I hate guest vocals is when they're done simply to market a smaller band on a larger band's song, or to vice versaly use Star Power to shine light on a smaller band's song.? For instance, and humorously enough, they both involve Amy Lee of Evanescence... when Paul McCoy (I think that was his name) of 12 Stones appeared and just yelled incoherently a little bit on their Bring Me to Life song, and when Amy Lee appeared on Seether's "Broken."? Boy, that was just awful.? Another example, and it makes me sad to reference this, is when Doug (dUg) Pinnick of King's X appeared on Dream Theater's Falling into Infinity album.? As great as both bands are and as interesting as Pinnick's backup vox were, it was just total Elektra interference and meddling.? I would've gotten away with a good album, too, if it wasn't for you meddlin' record label executives and your damnable dog!