Now that the VS 2013 Preview SDK is the owner of all project templates for VS extensions, including the “Visual Studio Add-in” project template, I have opened a kind of bug at Microsoft Connect requesting some reorganization of the “Extensibility” nodes in the New Project dialog:

Visual Studio project templates for extensions confusing in New Project dialog
http://connect.microsoft.com/VisualStudio/feedback/details/792892/visual-studio-project-templates-for-extensions-confusing-in-new-project-dialog#details