There is a book that attempts to piece together the ownership of the Lennon/McCartney/Harrison/Starr catalog called Beatles Songs(appropriately). It uses anecdotes, interviews, guesswork and black magic in order to attribute song ownership to the respective Beatle. From reading that book, spending 5 years immersed in a Beatles rabbit hole, and some other ways only legal in a few states I can say that a beatles song will mostly be sung by the main writer of the song. The major exception is the songs written for Ringo. The period around Rubber Soul is where things get murky as they were starting to assert themselves as individual songwriters and giving criticism/notes to each others songs. Where early beatles songs would have been a Lennon/McCartney writing collaboration of varying amounts, at that point John was the creative force of the band and wrote most of the original works. As Paul came into his own and started asserting his creativity, songs would be a collaboration after they were written. Paul was more of a producer with a great "feel" on how to make a song. From what I remember from the book Beatles songs, the author attributes the bulk of the song to Lennon, and gives partial credit to Paul. I think this seems about right. This song strikes me as something John wrote and was "punched up" by Paul. Coming from each writers perspective, they probably feel that they were more responsible. John may have written the song, but I seem to remember that Paul wrote the melody line on guitar, which is the main idea of the song.

I know people try to put each Beatle into a box and I they had their indulgent tendencies. However, I think they were really just 4 people that when put together were able to make music that few others could. Each of them has a moment where they are the weird, sappy, schmaltzy, psychedelic or rock n roll song writer. Its a lesson that I feel could also be applied to management when building a team. Find your "beatles". People that when you put them together create better output than each of them could alone.