I would tackle it the other way and ask when should an app actually be constrained to one or the other. Moving forward I would expect to make your apps rotation-aware and act happily either way. There are some circumstances where that really might not fit. For example in Android in Action, the chapter on Bluetooth and Sensors, we use an Android phone's orientation sensor to "steer" a lego Mindstorm robot. It would not make sense to allow rotation as you would run into problems driving the robot if the orientation kept changing.
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