When I attempted to create a Use Case diagram it was not available in the list (as shown below).   I found the following link instrumental to restoring this template.  I dropped to Visual Studio Command prompt and executed devenv /installvstemplates and waited (patience is required).  After it completed I had my template!  

http://msdn.microsoft.com/en-us/library/0fyc0azh.aspx 

Excerpt from the above link follows:

Available templates may vary according to Visual Studio version, SKU, installation options, and other customizations. If you are missing a template that comes with your installation, run devenv.exe with the /installvstemplates switch. For more information, see How to: Restore Default Project Templates.