i think you can still use an ios device with cable on a non bmw assist car. just the interface i think is different, and you dont get cover art and such.
in my opinion the only real thing you get is a2dp streaming and the actual on star like service that you have to pay for. if you have bmw apps and nav you get the google maps send to nav feature.
i'd try my ipod at a car at a dealer that doesnt have it and see if im happy with it. like i would have liked a2dp , but didnt get bmw assist since it didnt seem worth it for just that (i dont have an ios device though, so it had even less appeal).