In reply to Justin Tadlock.

Often when I browse new themes to install on a particular project I always use wptest.io data (and WooC if needed) to check if theme got what I want.
Then I fiddle around with customizer, I add new content with custom post types if theme has them. And If it is a theme that features page builder or some other type of layout management functionality I add a couple of templates and see what options and capabilities it has.

After that Reset and import of wptest content demo again. Deleting all posts and pages that I've created would be tedious task.

I know it is not a workflow that most of the people use but in my opinion theme library manager would be used especially by people that fiddle around with quite a lot of different themes. And fast import of demo data would be very handy, not as a core functionality but as a add-on.