Currently, halos (a sort of extra border) are only drawn on symbol maps. The cool part about halos is that for Shape Marks, the halos follow the edges of the transparent part of the image.
Although Tableau won't draw borders around Shape Marks, it will when the Shape Marks are on a symbol map. It would be great if Tableau could draw those halos around Shape marks on other mark types as well, so the halo could be used as a de facto border in other views.
Here's a pic of (orange) halos on blue circle shapes on a symbol map: