Add-ins for Visual Studio have become so irrelevant for Microsoft that apparently they don't even deserve a post in the Visual Studio blog to announce that they are officially deprecated in Visual Studio 2013. So I only became aware through this thread in the MSDN VSX forum that points to this MSDN documentation:
Automation and Extensibility for Visual Studio
"Visual Studio add-ins are deprecated in Visual Studio 2013. You should upgrade your add-ins to VSPackage extensions."
That means that at some point the Add-In Manager will be removed from Visual Studio. This was expected as I posted here and I have already planned to move my MZ-Tools add-in to a package as I announced here, along with a new series of articles about creating packages (next year) once I learn the stuff more deeply.