I have seen an earlier question titled "Shared Links/Shared Files in TFS 2010". The answer in this indicated that he was correct and there are some situations where a "VSS" style file share was needed. "Branching/Merging" and"Add existing linked file" do not cover all situations. Given that TFS 2012 has now been release I assume that such asimple and useful function has been added, but I can see it. Can you let me know where it is?
See http://connect.microsoft.com/VisualStudio/feedback/details/525330/shared-links-shared-files-in-tfs-2010 for further information