Yes, listening again after so many years Cannonball does seem the weak link. As was revealed by his leader dates, he isn't playing his natural style here, but seems to be adopting something of Trane's sheets of sound, which doesn't suit him. My views on this haven't changed much. When I heard the album on its release I complained to fellow student Jack Massarik (later to be jazz correspondent of the London Evening Standard) that Miles had picked a tasteless altoist. In those days Miles + altoist = Bird, where I was concerned.
I agree that "Billy Boy" is a banal tune, but for me the track is rescued by Philly's fantastic exchanges with Garland.