The CMS provides a lot of chrome, features and functionality that are not in platform (eg. control panel, user management, etc.).
The Framework includes all the underlying structure including database functions, string manipulations, etc. but it isn't a CMS. (According to the Framework project, the Joomla CMS will be refactored to use the Framework functions, but currently uses only a small set).
So if you want a CMS, there's no reason to migrate to the framework (unless you want to re-invent the wheel). If you want to build a web app, web services or command line application using proven functionality, you can use the Framework to provide lots of convenience and utility functions, much like Zend.
Here's a thread with more information: What are the differences between the Joomla! CMS, the Joomla! Platform and the Joomla! Framework?