For example on https://secure.phabricator.com/project/board/9/query/dFDJIh.T4tye/ the "Diffusion" tag is already not shown because it matches everything shown – but this is also true for "Files" but it's still shown.
When there are a number of different tags present, it gets a bit cramped; it'd be nice to waste as little space as possible on things we already know. :-)
This is probably at least somewhat reasonable, and we do something vaguely similar in the Maniphest list view.
It might be sort of nice to show that we're filtering these tags out somehow to reduce surprise/confusion, although maybe it wouldn't be too confusing/surprising, and this might just be odd since it seems sort of random.
(This also gets a little bit tricky with typeahead functions like any(...), viewerprojects(...), projects(user), and possible exact(...) in the future.)
Yeah, it was complex enough that I didn't want to suggest an outcome design too strongly. :-)
Regarding the space tags take up on workboard cards - would having tags without names (just icon/color) be hiding too much?