I notice that iTunes 12 automatically writes metadata changes back to the file. Unfortunately, if you are using TimeMachine to backup your music library, this is a huge waste, since it means that every time a file is played (or any other metadata change is made) a new copy of the file is included in the next backup.
I understand that the file metadata is stored in ~/Music/iTunes/iTunes Library.itl. Even with nearly 10k songs, this file is less than 5MB. If a file is already in the iTunes library, I believe iTunes no longer considers the metadata in the file.
I remember in previous versions of iTunes, it did not automatically write metadata changes to the file, but this had to be triggered manually. Since I am not likely to migrate the files out of my iTunes library any time soon, but am more concerned with avoiding file churn, how do I stop this?