I agree with most of your point, and I think not only with the contractor but even sometimes with internal department and even members from the same team. You need a kind of documentation and often it is hard to document or prove a verbal agreement. Written agreement always a good option to protect your self as well. I had experience that some of my team members say something and denied it later. So I start to ask for email even after oral discussion. And that’s why meeting minutes Also important because it is considered as a written proof. SO same idea for contractor or agreement with companies or partners. it is always good to have a written agreement in my opinion.