It was not there 4 years back, when the iOS was just beginning its life. Whether you like it or not the truth is its fragmented now. There are all different versions of iOS, then different versions of the devices. These two itself bring a lot of different considerations, while developing for the platform now.
Its agreed that its less fragmented but then thats enough to bring in the pain to a frontend developer.
Now, to add to it, there are different device form factors. Starting with regular iPads, retina, air, mini, iphone, ipod touch and all have different specifications. So the point is, the XCode workflow allows one to design for all different devices, that does mean design is multiplied not reduced. While its good and freedom to the UI designer, as a frontend developer one must keep an eye on all these form fragmentations.
Now the same code will produce, a different looking back button in an older iOS and a different looking one in the newer ones. The UI may hide some UI elements on a smaller heighted device and look perfect in a newer bigger iPhone/iPodTouch.
Thats doable but definitely increases time to implement UI, if its not designed, and well thought of, by people, who know what they are doing.
Happy designing.