The same reason you started the ellipse visualizer to being with.  You could make an ellipse now which has an outline and a fill using the Polygon and Polyline properties.  They even share the same vertexPositions, which means there's no data duplication.  I assumed the reason you wanted an ellipse property is so that you could easily define an ellipse and control it's bearing over time.  If it's just a static object, you could just use polygon and polyline.