Furthermore, the Beatles' early bass player was not Paul McCartney but Stuart Sutcliffe, who by all accounts was the least talented performer of the band. Stu's lack of ability as a performer limited the band's repertoire. Howie Casey, saxophonist with the band Derry and the Seniors with whom Stuart played in Hamburg, said of the bassist, "All we could do with Stu was to play twelve-bar blues. He couldn’t venture out of that" (Sutcliffe page 88). If the Seniors couldn't play anything but 12 bar blues because of Stu's limited facility on bass, no doubt the Beatles couldn't either.
Even when Sutcliffe left the band and McCartney (a much more gifted musician) assumed the role of bassist, the Beatles' retained many 12 bar blues tunes in their repertoire, several of which wound up on Beatles records. As the band progressed and their musical abilities developed, however, their reliance on the formula decreased, replaced by their own unusual and strikingly original harmonic progressions. Certainly by 1967 the 12 bar blues was ancient history from the band' perspective. Referring to the orchestral passages in "A Day in the Life", Paul McCartney said, "It was very exciting to be doing that instead of twelve-bar blues" (Anthology page 247). (Odd that Paul would say that regarding 1967 when the previous year the Beatles released no songs whatsoever that incorporate the 12 bar blues.) During the three years from 1963 through 1965, the Beatles released 19 songs using the 12 bar blues or something comparable; during the five years from 1966 through 1970, only 8.
Beatles. The Beatles Anthology. Chronicle Books, San Francisco, CA, 2000.
Sutcliffe, Pauline and Douglas Thompson. The Beatles’ Shadow: Stuart Sutcliffe & His Lonely Hearts Club. Pan Books, an imprint of Pan Macmillan Ltd, London, UK, 2002.