Why isn't the DefaultItemRenderer class inherited from Skin?

Question

\sdks\4.0.0\frameworks\projects\spark\src\spark\skins\spark\DefaultItemRenderer.mxml
<?xml version="1.0" encoding="utf-8"?>
<!--- The default skin class for a Spark DefaultItemRenderer class. 
      @langversion 3.0
      @playerversion Flash 10
      @playerversion AIR 1.5
      @productversion Flex 4
-->
The DefaultItemRenderer.mxml is skin class for ItemRenderer.cs,but isn't inherited from  skin?

Answer 1

And to clarify one more thing, we also thought about making item renderers skinnable components (i.e.-ItemRenderer extends SkinnableComponent).  This way, everytime you were changing the look and feel of a component in Spark, you'd be following the exact same pattern.  But after we thought about it some more, that SkinnableComponent-Skinning model didn't really feel right here.
With a SkinnableComponent we have a clear separation of the component properties and behavior on one side and the look and feel of the component on the Skin side.  Also, there's a clear contract we use to talk back and forth to one another.  The reason for the separation between the Skin and the SkinnableComponent is so that we can have one Button SkinnableComponent and multiple Skins for that Button which all tweak the visual appearance of it.
It doesn't make sense for every component to be skinnable.  If you know what you're component is going to be and look like and don't need the extra flexibility skinning provides, then you can get rid of the extra overhead that skinning requires (like having 2 classes).
With an ItemRenderer, we don't really need that separation and extra complexity.  It's really rare for someone to want to create an ItemRenderer component for a particular piece of data and want to have a separate Skin file for that ItemRenderer's look and feel.   For item renderers, the user really wants to create a custom item renderer with any extra behavior and the look and feel of the component all baked in to the same class.  Because of this, when you're creating a custom item renderer, you extend ItemRenderer, like:
<s:ItemRenderer focusEnabled="false"
    xmlns:fx="http://ns.adobe.com/mxml/2009" xmlns:s="library://ns.adobe.com/flex/spark"
    autoDrawBackground="true">
    <s:Label id="labelDisplay" verticalCenter="0" left="3" right="3" top="6" bottom="4"/>
</s:ItemRenderer>
In general, if you want to build a custom component that isn't skinnable, you'd probably just extend Group (similar to our custom item renderer) and bake the child components and look and feel of the custom component right in.  If you want more flexibility and to build a new skinnable component, then you'd extend SkinnableComponent and create a default Skin for its appearance.
Hope that helps,
Ryan