I would agree that you should not train with the intention of fighting, but to learn how to avoid a fight. That said, I believe that it is vital to have an instructor in the room when you are training, as too many aspects of the martial arts defy video instruction.
Let us consider the foundation for your martial art, your stances...if your stances are not done properly everything will be at least a little wrong top to bottom, and stances are a thing that comes with years of practice in front of a mirror, and in front of an instructor.
Moving in and out of stances with fluid motion and power is another thing that I do not believe I could have grasped without my instructor watching me and correcting my mistakes. The subtle moves involved in keeping your hips in line, and staying balanced while moving in stances is not something that comes quickly, or easily for many.
Lastly I believe a part of having self confidence when faced with defending your life, or the lives of your loved ones comes from experience in contact fighting. Learning that getting hit in the nose hurts but only for a little while, that while a hit to the gut hurts it wont kill you...the realization that if set up in combination with movement and punches a front kick actually does floor an opponent...for me these are things I learned in the ring. This type of fighting can be done on your own I suppose if on a video course, but not safely.
Most black belts I have know consider a white belt to be the most dangerous fighter in the room, for a general lack of control, and almost completely unpredictable actions when fighting...this is where supervision by an instructor saves people from broken bones...adding an element of control to the chaos of a new fighter.
Take this how you will, I do not mean to insult instructors who sell video training courses, I consider them to be a valuable tool in learning a martial art. I do not however hold video training courses in very high regard as a stand alone tool for learning, the function of the sensei, sifu, master, whatever you call them in your art, cannot be replaced by a video course.