Andrew66,
The reason I use primer is for the panel lines and rivets. Once I have the WBPU and talc sanded down, I draw all the panel lines with pencil. Then I put chartpak tape over these lines. I spray 3 coats of primer over this tape and when you remove the tape you have panel lines. The primer is also good for showing any imperfections in the WBPU/talc coat, and it is the layer which I burn all my rivets into. I think you are right though Andrew...you could just as well paint right on to the WBPU/talc layer.