A primitive and an invisible wall does the same thing
For as far as I understood, primitives can be moved at runtime which is / was impossible for walls. Please correct me if I'm wrong 
What exactly are you using the primitive for?
For flipper logos there is a flipper under the primitive.
For drop or stand up targets a wall works fine.
For a toy such as the trunk in theater of magic a wall works fine.
I dont know of any other toy on a table that interacts with the ball in which you couldnt use a wall.
Can you give me an example of what you need.
Walls are dropable so you can move a wall with the primitive.
I think the issue with making a primitive collidable would be that for a lot of the toys the images used to make them have transparency so the entire primitive doesnt make up the toy. Look at the xwing in jps star wars. So the ball would actually appear to collide with nothing in some cases.
Its not like the models in fp. The x wing in star wars is actually an image on top of a square box
I hope that makes sence