Why Ruber's Villain Song looks more like a mediocre poem. The guy is completely and utterly insane, so of course he wouldn't be able to construct a proper song.
Devon and Cornwall get a tiny example — maybe — following their weird duet about wanting to be separate. Garret reluctantly allows them to come along on the condition that there be "No singing!" Devon inquires how Garret feels "about interpretive dance." The man is blind. The question mark for the Fridge Brilliance lies in whether or not Devon was aware of Garret's disability at the time.