This model won't include any athlete identifier, which makes the information a bit useless in the end. Since APIs are made for machines, and machines identify entities differences between IDs, not having them on the returned models make them invaluable.
It would be nice if ClubAthlete had the athlete ID included within its definition. This way, apps for clubs, built on top of the Strava API, can easily map and identify the athletes. Otherwise this is impossible, as there's no other way of identifying athletes related somehow to another one who granted access to the app.