Absolutely NO addons should be allowed in the game. It's a doorway for cheaters, bots and gold sellers to destroy the game.
Make a highly customizable UI that we can change and toggle elements and there's no need for addons.
I'd also make the same argument for Macros and API's. (API or Application Programming Interface, is a software intermediary that allows two applications to communicate with each other.) Addons, Macros and API's all give nefarious people options to corrupt gameplay in some way.