Additional training courses have particular objectives and allow you to learn the required skills to dive safely. All courses, regardless of the certifying agency, are developed through many years of experience. You’ll learn in many different ways, by reading, talking, watching, testing and doing. Plus, you’ll earn a nice certification and id card :) AND, it’s hard to show others that you’ve obtained a certain level without having the id card to back it up.
I suggest the additional training.