Not true at all. You tie your risk into multiple factors that can go wrong.
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.
Not necessarily. Yes, the comparative advantage of roles playing to their strengths can net you a lot of short-term gains, but you run a tremendous risk of getting stuck if one person quits or get hit by a bus, for example. Knowledge sharing via cross pollination of roles is a good risk mitigation strategy.
Yes but if you had it split into three experts for each area the chance of they all getting hit by a bus?
Also I think if you hire an expert in field x, there is a lot higher chance that their coding style and practice will be easier for another expert in field x to pick up. Ever seen a developers photoshop files :p