If you trust Apple with what they do on your iPhone, you should trust the third-party with what they do inside their app, there is no difference.

Why is trust an all or nothing proposition? Why can't I trust an app to do whatever it wants within its sandbox, but not to get an unique identifier that can be shared between apps? It's not any different than sandboxing, where I trust the app to do whatever it wants in its sandbox, but not mess with my documents or OS.