The typical I-IV-V-I chord progression became a virtual institution of early rock music. It can most commonly be heard in early rockabilly music from artists such as Buddy Holly, Chuck Berry, and most famously, the early works of the Beatles. This pattern, however, soon shaped an evolution from fun, youth-like rockabilly to the hard rocking sound of the late 60s and early 70s.
In the music of The Beatles, this "borrowing" of chords from the minor mode while in the major key makes itself evident as early as 1963 with the release of their first album, "Please Please Me." Many of the songs on this album were covers of early rockabilly songs from artists including Carole King. However, the song with which the title of the album shares its name is a Beatles original, and possesses the usage of a new mixture chord within the typical I-IV-V-I progression--now, instead of this progression, the Beatles go (in the verses) from a sustained I to a quick IV, back to I, and then, surprisingly, a bIII - IV- V! The mixture chord here is apparent; it is the use of the bIII. This chord change doesn't occur while John Lennon is singing--John finishes his sentence "Last night I said these words to my girl" (while the I-IV-I progression occurs) and after he stops singing, the bIII makes itself known in a sort of short following statement from the guitar. This bIII brings with it a few implications to the listener as well as the performer.
To the listener, it is reasonable to expect that a bIII would not show up in the upbeat, happy I-IV-I progression that has already been established. Therefore, this bIII is a bit of a surprise, but not in a shocking or undesirable way. Perhaps in this song, it is important that Lennon is not singing while the bIII is played. Perhaps the guitar statement can be seen as a quick interlude to give the listener time to think about what it is that Lennon said to his girl the night before--because the audience is so intent on listening to the words, perhaps they will also notice the interesting modal mixture in the instrumental interlude, though he/she may not realize it. This gives the listener a chance to hear a new type of chord progression and focus on a type of raw sound that had never been created before. This funky bIII from Harrison's guitar provides a driving force to the song that it may not have had if replaced with another typical rockabilly chord. Because it was so much different than anything the youth of the early 60s would be used to hearing, it provided somewhat of a new sound and a groove that would open itself up to even more musical possibilities in the future. The Beatles realized this and soon capitalized on modal mixture in future songs including Help!, Strawberry Fields Forever, Magical Mystery Tour, Sgt. Pepper's Lonely Hearts Club Band, and Oh! Darling.
For the performer, this mixture creates a bit more of a hard rock type of sound than in past years. For the Beatles, this was something different than what anyone else had been doing, including their own work. For them, that meant that making the usage of mixture chords more predominant in their work could certainly be to their advantage. Looking back on it now, it is fortunate that they did just that--from Please Please Me to Abbey Road was certainly a dramatic evolution of rock music that couldn't have been shaped without the use of modal mixture.
To read more about the prominence of mixture chords in the music of The Beatles, see this book, The Songwriting Secrets of The Beatles, by Dominic Pedler.