I found this article on a blog called 'Where's that sound coming from?' and I thought it was quite interesting :
Its title was "What's in a Voice? A light pondering of timbre and its inequities."
"Lennon vs McCartney:
This pretty much sums up the basis of this thesis. There is the conventional, and very inaccurate, opinion that "Lennon is sincere, smart, witty.." and "McCartney is glib, insincere, all craft, song-and-dance and no emotion". Why? Timbre! Lennon was blessed with one of the coolest, if not the coolest singing voices in pop history. He could roar and yowl, make you feel his pain and send shivers down your spine ("Yer Blues", "Anna", Don't Let Me Down", "I Want You (She's So Heavy)"; he could command your full attention and control your emotions when in a thoughtful mood ("I Don't Want to Spoil the Party", "You've Got To Hide Your Love Away", "Norwegian Wood", "Julia"); and he could spike your drink with whatever he was taking that week ("Strawberry Fields", "I Am the Walrus", "Tomorrow Never Knows"). Listen to "Dear Prudence" and you just feel blessed afterward. You feel Lennon's presence.
On the other hand, Paul McCartney, when yowling ("I'm Down", "Helter Skelter", "Oh Darling") can sometimes make one cringe with discomfort. When singing a cute song ("When I'm 64", "Maxwell's Silver Hammer") Paul seems very at ease, which lends credence to the prejudice. Lastly, when singing a serious and emotional song ("For No One", "Elenore Rigby", "She's Leaving Home"), Paul seems distant and cold--he's reporting, not baring his soul.
So let's pretend Paul sang "Being For The Benefit of Mr. Kite". It'd be just another annoying, cute Paul song about the circus. But John's timbre makes it seem like a rather scary circus, even if he didn't want it to be. If John sang the beautiful "Mother Nature's Son", it'd be seen much more as a deep philosophical meditation than a pretty singalong. It's the Lennon/McCartney double standard that still seems to haunt Paul."
Source : http://wheresthatsoundcomingfrom.blogspot.com/2011/02/whats-in-voice-light-pondering-of.html