And remember, any trick abused to create tracking IDs stands the risk of being detected and blocked by Apple in the next iOS update. It has been announced, it has happened, it will keep happening.
That's right, but it does allow persisting an id through reinstalls of the same app, and sharing that id between apps of the same developer.
> So no, this does not allow an tracking ID usable by all apps.
It effectively does though. For example, imagine a mobile game company with 10 games. With this technique, you can track that user across app re-installs in each of those games.
Now imagine another game company doing the same trick. If both companies send up their independent tracking IDs to a central server along with any other info they can get about the user (email, screen name, IP, whatever), then you can strongly correlate users across multiple tracking IDs. The user has no way to reset these IDs even if they delete and reinstall the apps using them.