personally, i'd prefer not to have a drop down allowing to filter the styles by usage...
(i know that you can find similar options in office... but i always have the feeling that it's a top-heavy over-engineered feature...)
i'd prefer
- a "delete unused styles" button
- that shows a dialog asking for confirmation,
- with a list of the styles that will be deleted,
- with a checkbox that lets you uncheck the styles that you want to keep even if they are unused. (maybe with a revert selection button...)
this + a way to import single styles from another document seems to be more powerful to me and does not clutter the UI with even more information that is only rarely used (it can be an important information, don't get me wrong!, but most of the time, most users are not interested in that information...)