The Value Filter is useful for displaying an itemized list of vales, for a given metadata tag, from which I can filter for any # of those items. How to do the functional equivalent thing with a Reference Attribute? The Attributes Filter doesn't offer filtering on a Reference File Attribute, only searching.
This is how it is implemented. You can filter for any Attribute value, including all reference values.
There is no filter which enumerates all values you may have used for an Attribute if this is what you are looking for.