> ** Allows the application to modify the state of any component in the tree
> just before the tree state is written out. Does that seem useful?

I have never had a use-case for a callback after rendering and before
saving the state. Why would that be necessary - a concrete use-case?
If I create components dynamically, I make them transient, so they are
not save-stated anyways - so no need to modify the state of components
before save-stating.

I'll agree it is an edge case. Originally when I created the secure token tag for Seam, I wanted to add markup to the encoded HTML and put the token in the tree state. I decided instead to do that work during encoding in a component tag. The downside is that I couldn't add the functionality to existing pages without modification.