George Martin used to quibble over whether "The Ballad of John and Yoko" was even a Beatles song. What's certain, however, is this: Without it, the group might never have rebounded from the crushing disappointment of Let It Be to complete Abbey Road.
"It was hardly a Beatle track," Martin said in Anthology. "It was a kind of thin end of the wedge, as far as they were concerned. John [Lennon] had already mentally left the group anyway, and I think that was just the beginning of it all."
Something happened on April 14, 1969, however, as Lennon and Paul McCartney worked feverishly to complete this new track: The scars from their most recent sessions began to heal. Martin was back at the helm, after stepping aside for Phil Spector on Let It Be. Engineer Geoff Emerick also returned after having departed during sessions for 1968's White Album.
Source: ultimateclassicrock.com