@BetterRed, thanks for moving it.
I hope someone would be interested in that (I posted it on launchpad before:
https://bugs.launchpad.net/calibre/+bug/1516186).
There is a number of uses for emblems in book details: marking read books and books to be read, favorite books, book on the device, language of the book, and even things like genres if someone feels like that and finds graphics small and clear enough.
All in all, one could use emblems pretty much the same way as in the grid view or as icons in the standard view, with one difference: book details offer much more space, so that's where you could get really creative without cluttering the view.