If I have a bunch of ViewControllers that are only ever going to deal with a single view, or even if my ViewController is going to deal with multiple views, why should I use another ViewControllers to manage the other ViewControllers? Why wouldn't I just change out ViewControllers at the ApplicationDelegate level?
Maybe I'm thinking about ViewController the wrong way? I'm used to writing in the MVC pattern with Ruby/.NET. For an example, if I were working with widgets I'd probably have a WidgetController and a List view, and a Detail view for the WidgetController.
What is the analogous iPhone MVC construction? I'd assume the WidgetController would subclass the ViewController and I'd have a couple different views depending on how I wanted to look at the widget data. Then, when I wanted to deal with Wodgits I'd make a WodgitController with its associated views and swap the window's subview out with the new Wodgit ViewController.
I don't see what having a RootViewController to control my controllers buys me. Where is the value? What am I missing?
I'll focus on one aspect: Compartmentalization is a very useful design principal. If you separate concerns and put things where they "belong", they will be easier to find, maintain, build upon, and co-opt for new purposes.
Consider this:
All the pipes in your house go to the same place. Do you urinate in your sink, or do you use the fixture designed for that purpose?
In the case of your app, the application delegate is responsible for responding to application events and managing universal application resources. The root view controller is responsible for managing your views and view controllers. As a view controller it has special abilities to do so with navigation control, the ability to present modal views, act as a delegate, etc. While you could create a view controller and manipulate it completely in the application delegate, the task will become exponentially more difficult as you multiply the number of views you are managing, especially if you wish to be a text field delegate, data source, etc. On the other hand, it is easy to split the root view controller into its own class. There is very little cost.
Why pee in the sink when the toilet is a step away?