Great producer and arranger. He also composed the orchestral soundtrack for Yellow Submarine.
What a mess The White Album is. Such excess, such waste.
I suppose they thought they didn't need him anymore...they knew it all. Which is why the White Album is mostly crap with a few good songs here and there.
Early in their recording careers they listened to Martin when he told them a song was either bad or needed redoing. When they ditched him they allowed their rather large egos to make decisions for them.
Now, it is possible they simply ran out of good ideas by the time they split up. But I would bet getting rid of Martin didn't help. Certainly, McCartney's mostly crappy albums post-Beatles could have used Martin's musical expertise.
It's ironic that the only Beatle who I thought had consistently good music post-breakup was Harrison. He was the Beatle whose music Martin relegated to third place during the Beatles reign at the top.