Is there not an easy way to just say "turn my app upside down when the user flips the device"? For most apps (i.e. ones that don't use the accelerometer, etc.) this should be trivial, and shouldn't require any real effort from the app developer.
Also, I agree with Apple that both orientations should be supported. Depending on how you're holding or docking the device, and where it's convenient for the headphone jack to protrude, it's quite conceivable for users to want to use both orientations.
Assuming you programmed your app correctly, you only need to override the function shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation to return true for the supported orientations. If you use portrait orientation, you can get portrait upsidedown for "free", as it rotates the view for you. It's the same with LandscapeLeft and LandscapeRight. The only complications come when you want to support both portrait and landscape orientations, which isn't required anyway.
It's trivial to support two orientations, but Apple had (has?) a guideline that said you must support all four orientations. That's not necessarily trivial, although Apple's UI kit has features to help you there too.
Also, I agree with Apple that both orientations should be supported. Depending on how you're holding or docking the device, and where it's convenient for the headphone jack to protrude, it's quite conceivable for users to want to use both orientations.