It's easier to replace one entity with another, than several parts.
Knowing the SDK and design elements beats a designer that only knows design.
And the API consumer would probably develop a better API than some guy who generally doesn't consume most of his API for a living. That being said, that assumes the front-end guy also knows back-end. Rare, but exists. points to self
I generally dislike "Mobile UX, UI, IOS Design expert" if they can't code their design. Either for Cocoa or (S)CSS CS/JS depending on platform.