I've watched a lot of interviews with Rick and about Rick, and his role seems to be entirely limited to being a muse for artists. He's not technical, he's not a musician, he doesn't really do anything beyond taking naps in the recording studio while the musicians record, occasionally opening his eyes to ask them how they feel.
To be honest, it would almost be possible to argue that Rick Rubin doesn't do anything worthwhile if he didn't have an outrageously long and unimpeachable resume to back up his contributions.