This is IATF is it not? If this is a discussion about IATF, then VDA 6.5 is relevant because is both the document that provides the guideline for Product Audits and it is the methodology expected by some of the automakers and their suppliers. In my particular case we have now had over a dozen audits using VDA in the last 4 years.
Sure an organization can make up its own process, but that simply makes no sense. "We have the best recipe in the world for xxx, but let's just make up our own." I prefer to follow a good document that is pretty easy to follow, makes life easy, and makes customers happy.