Sometimes it is impossible to create needed representation rules based on geometric effects and marker placement options. For example cartographic symbols in ex USSR( Russia, Ukraine and others) on cartographic maps like slopes, rocks, stairs.
The only way to implement them is scripting. Today we have to write commands to render these symbols with free representation. I think it would great if ArcGIS had the possibility to add custom representation rules defined with Pyhon scripts additionally to geometric effects and marker placement options. So there is no need to create ICommand in С# for drawing complex symbols with free representation, and it would be transparent for end users.