I see different accounts. Many imply his brother "cracked" the case by fingering him as a suspect, not some FBI statistical wizardry. Perhaps stylometry added to the evidence, though.
As far as The Beatles, a large part of their success is the juxtaposition of contrasting styles, in my opinion. Lennon had a sarcastic style and McCartney a more optimistic sound. Too much Lennon is whiny, and too much McCartney is syrupy and cheesy. Mix and/or juxtapose them and you get a great balance and contrast: flavor AND spices.