I'm suspicious that Billy Preston may have been a ghost writer of "Let it Be" and maybe "Get Back" and "Blackbird", just a gut feeling. Basically he was a church guy, and Let it Be is a typical church song, untypical of The Beatles, and his billing as "The Beatles with Billy Preston" on records seems to be an over compensation for his piano solo on Get Back- he gets "Billing" instead of a performance credit
"I Wrote A Simple Song"
I wrote a simple song
With simple words and harmony
Wasn't very long
Before a star, I was bound to be
I didn't care if it made the charts
I only wrote it for you and me
They think they're so smart now
They're not as smart as they want to be
They took my simple song, yes, they did
They changed the words and the melody
Made it all sound wrong, yeah
Now it sounds like a symphony
Who told them to mess with my?
Oh, now, who gave them the right?
I wanna know now, that song was personal
Because I wrote it for you, it's yours and mine, girl