Not sure what you're getting at here. In our organisation, we only have one Public NAB, and the CDC contains everything from there. So searching the Personal NAB then the CDC is exhaustive for our purposes.
If you're hinting that you may have other address books to search (cascading, Directory Assistance etc) then you can set up the CDC to index those too.
I take the point about ambiguous lookups when users copy Public NAB entries to their Personal NAB, but that can any time, irrespective of whether you've got a CDC or not.