I've got a View with an exposed Date filter using operator "Is equal to", granularity set to Month, and the Optional option enabled.
I'm not sure if there's supposed to be an "All"/"Any" option added to the year and month select lists when Optional is enabled, or if the "-Month" option is supposed to handle that (perhaps not very intuitive if so?). If the "-Month" option is selected no nodes show (it does filter properly if a month is selected).
indicates that in 6.x-2.x-dev with exposed From and To dates it works (using -Year or -Month, etc as the "any"/"all" option). My view only exposes the From field.
You can see an example at http://harmonycentrefoundation.org/calendar