True but, IMHO, a lot of the higher level task should be able to handle it. I guess you can have a simple real time IC to handle the flight envelope to provide stable flight and then use the iPhone to do the advanced operations. For example, if you are building a drone that is mapping the environment and follows you through a bike ride, does it really matter if the 3D environment it creates for autonomous navigation is slightly off? You can continuously compensate for it, stick with moving averages and avoid extreme moves.
That real-time controller still needs the IMU sensors, which is maybe what GP was responding to. (Your iPhone could have an additional set of them, but the ones on the phone don't do any good to the real-time subsystem.)
Oh, I got an accelerometer card for that. It's very small and cheap, there are also cards that contain a gyro + barometer. So if the phone ones are not real-time enough cheap options exist.
Hence a microcontroller for flight control. The iPhone can just decide on the higher level tasks and send control signals to the microcontroller, similar to how a microcontroller delegates tasks to the underlying hardware peripherals.
