This is true but this only helps on the map, not on the cachelist below the map. On both HiddenDate and HiddenMonth, the list gives a very good overview as it shows the dates/months directly in the list. To get this information on the map, you have to click each individual cache. So personally I often prefer the list.
If it's not possible to include the "found-filter" on the pages, would it be possible to mark the individual caches on the list below the map with a smiley if they have been found?