Nested or not nested? That is the question

Hello,

I'm working on a implementation of dfxp, and i would know what is the  
current status of nested div and nested span.
Actually, the recommandation says that the Core Vocabulary does not  
support them, but the schema was changed back.

And I do not really understand what is the purpose of nested div and  
nested span. Is there an example of how they can be used in real life?

Thanks

Jean-Daniel Dupas