I'd say he actually contributed to the creative process of many songs. Suggestions in recording studios have synergy or not. Paul called him after they produced recordings elsewhere. Paul wanted the Beatles to record together like they use to do. Like Sun Studios in Nashiville, somebody contributed something very special during the recording process. Great stuff came out of there. Jimmy Page was a great studio musician and brought many recording skills to Zepplin's sound.
Martin provided a balance and themes to albums in terms of some song selections, too. The alternate takes on songs while making an album had many of his suggestions for musical coherency. His earliest mono recordings are historical, and the latest DVD-A Love is beyond any remix I've heard lately.
