The NDA is a great mechanism to protect the company during partner ventures, but not a substitute for a patent. For example, you find out that company ABC (to whom you never told your idea), has put out the same idea that you had. You cannot prove how they found out about your idea, but it does not matter. If a patent is in place and it is clear that your product is covered, you can sue them in the court for patent infringement. When you have a patent, even people who innocently and coincidentally come out with the same product could be held liable for patent infringement.