The Voice, being the most nicey-nicey of TV's singing competitions, doesn't give its coaches many opportunities to rail at terrible singers. This can get stifling. It's the sort of thing that makes some people look elsewhere for chances to be creatively cruel. It drove Cee-Lo Green all the way to the Late Show with David Letterman to get his diss fix.
The jokes Cee-Lo delivers are Letterman material, mind you, so the quality of the joke writing's about the same quality as the Muzak-y "Fuck You" rendition that sees Lo in and out. (And you just know the production notes called it "Forget You.") But as anyone who's paid even the most cursory attention to pop culture knows, nobody turns a phrase quite like Cee-Lo Green. It's something about the stare, about the stage presence. It's something that's served him quite well throughout his career. So just face it: you're watching the whole countdown.
(Warning: truly gruesome singing at 2:17.)