We decided to make the move to Community Server because of the seamless integration of Blogs and Forums, as well as the ability to easily integrate a Single Sign-On (SSO) solution.  During the transition, we also found that integrating the Customer Portal was a possibility and made some changes to the Customer Portal code to allow for it -- which is now available to all OnTime Customer Portal users (and does not require Community Server)

I guess in short, the main reason was added features and functionalities that Community Server brought the table, which are, simply put, the ability to build a community.