From our experience most of our customers do in fact need primer and it's something that's frequently overlooked. So when is primer needed? The answer is pretty simple. If you're touching up an area that has exposed metal or plastic (basically any exposed substrate) you need primer. If you are planning on using any type of body filler you need primer before applying base color.
The only time primer is NOT needed is if the damage is light enough only the clear coat or base color is affected, thus leaving the primer coat intact. Most light scratches and rock chips fall in this category.
Adding primer to an area that doesn't need it won't hurt, so we always recommend adding it if you're unsure.