I found the below in Google regarding the Synchronization of Metadata by Micahel Pisarek http://www.sharepointanalysthq.com/2012/06/documents-sets-vs-folders-in-sharepoint-2010/
My question is does the synch of Metadata impact all existing values or can I manage it so it only changes only new documents. As an example if I have a metadata field of Project Manager and until 10/31/2013 Joe Smith was the Project Manager, but after 10/31/2013 Lucy Smith was Project Manager I assume it will change all documents to one value only for Project Manager.
Synchronization of Metadata by Michael Pisarek The ability to synchronize metadata allows users to change metadata on multiple items at once within a Document Set. Frequently users will want to do a bulk change of metadata stored within a folder. The most common way to achieve this using Folders is utilizing the Datasheet View for document libraries which allow users to bulk change properties quickly. However this can be error prone and has limitations such as the inability to change Managed Metadata fields (see more limitations here) In contrast Document Sets allow the ability to configure a column as a Shared Column. Shared Columns then share metadata across the entire Document Set. By changing the column value at the Document Set all content contained within the set will be updated with the new value without the user manually having to change each individual document. Another advantage is that all columns types are supported, so users can also easily change Managed Metadata columns for all content as well.