Exactly. I rather suspect that there are some parts of UIPopoverController that have problems with the different screen sizes on say an iPhone 4S and an iPhone

Exactly. I rather suspect that there are some parts of UIPopoverController that have problems with the different screen sizes on say an iPhone 4S and an iPhone 5S. Apple trust themselves not to call the parts that don't work, but have not yet ironed out all of the bugs for third-party use. Unless I've missed something about UIPopoverController, it's implemented using public APIs, so I'm curious as to what the author thinks Apple's motivation is for holding back this API - it's not like you can't create the same functionality yourself using existing APIs, it's just a bit more work. My guess would be that maybe they don't want to see people using popover controllers all over the place on the phone, so they made the bar higher for implementing such functionality.