I also use a fine grind. I grind my own coffee, 8 O'Clock Columbian beans, and as said I find it does add an earthy flavor. I will say it just adds another layer of awesomness to the flavor.
Steve you don't add alot, like I said about 1/2 tablespoon per rack of ribs. If I'm doing a butt 8-10lb range I add about 1 1/2 tablespoons to the rub