The problem with guaranteed goals is that some people see they have to farm X amount of Y and feel they must farm for it every single day. While with RNG everyone has a point were s/he looks at the chances and says "fuck it I'm only farming it when I'm in the mood".
I liked the Cata model of valor points best. You could cap it with 7 dungeons but it didn't matter when you did them. IMO better then giving a daily bonus for the first dungeon.