Updated:Paul McCartney did his best Kurt Cobain impersonation on Wednesday night, joining Nirvana's Dave Grohl and Krist Novoselic for a spirited performance at the 12-12-12 concert. The special development closed the show, which benefits victims of Hurricane Sandy. McCartney first took the stage to perform "Helter Skelter." He was later joined by Diana Krall, with whom he performed "My Valentine." But eventually, it was time: Grohl, Novoselic and Pat Smear took the stage.