At the time UISplitViewController was invented (iOS 3.2) it was illegal to create your own container view controller, i.e. a view controller whose view contained the views of other view controllers. You had to use only the container view controllers supplied by Apple. So what UISplitViewController was doing couldn't be done any other way.
In iOS 5, custom container view controllers were introduced and UISplitViewController immediately became unnecessary; it became legal to write your own alternative. I even wrote one (warning, this is iOS 5 code):
So my advice is, as long as you understand how custom container view controllers work, do what you like!