You can point out that even Microsoft states in [this][1] article:
 

> Updating the code to improve features or to fix bugs would require
> that the Office artifact be re-emailed, re-structured, and re-worked
> by every single user and in every single file that has been using the
> customization.

The article is about different approaches of developing stuff based on Office, maybe you can take also some other pros and cons from it in your discussion.

  [1]: http://blogs.msdn.com/b/vsto/archive/2013/06/18/roadmap-for-apps-for-office-vsto-and-vba.aspx