I guess this is more an argument about designers without experience of the platform - but most designers /don't/ have experience with iOS. Especially in an agency model it's easy to "receive" a design that uses very slightly non-standard controls, attempts to use Android UI paradigms in iOS (or vice versa) or generally make everything a pain.

For example, I've had discussions where a designer has insisted that the navigation bar is 50px high, which is a proper pain. (The correct solution here being to leave it at the default of 44px, safe in the knowledge that they'd have to be properly anal to count up the difference).

Why waste time doing it on a photoshop canvas when a developer could easily and quickly get an overview with Cocoa and auto layout through storyboard?

Background: none.

Because stock iOS (iOS 7 changes this a little bit, but not much) only gets you so far. Things like pixate get you a little further, but ultimately some sort of visual design process comes into play, typically with a tool that's been used in that process since time itself began.

Background: 20 years as a creative technologist currently doing iOS based experiential retailing for huge brands where the interactivity and visual appeal simply doesn't exist in stock iOS. And I also wrote an app publishing tool that works similarly to Adobe DPS (build informational apps in Photoshop, see http://www.youtube.com/watch?v=YXlHFhbqzHU).