I would say the most important aspects of any martial training is in partner or multiple partner drills, training, and sparring. You will not learn proper timing and range without a partner but along with proper equipment like a punching bag or pad one can work on movement, structure, speed, power, tempering of the hands arms, legs and much more.
As an instructor I want my students, I encourage my students strongly to practice at home. Why would I? Do I want them to have better footwork, better balance, stronger legs, arms, bodies? We teach, instruct, and coach. There is a time for each and there is the time for the student to practice on their own. They will make mistakes, OK. A good teacher will allow them to make mistakes and then coach the corrections. The student will make the changes when coached properly. Having done something wrong and then coached to it being proper allows the student to know and understand what was wrong and why. Getting hit, kicked, taken down, thrown... all have their places in the advancement of the student's understanding and development. Practicing on one's own is a great place to get a lot of self-discovery about one's self and abilities and is a great tool to be used in conjunction with group instruction, one on one instruction, and one on one training. Practice as much and as often as you can.
I agree with what you are saying 100% but can see the issues arising from giving a student a DVD of material for that belt level when he reaches it. You would be in essence, giving him time to skip ahead of what you're teaching him and make mistakes that could cause an injury. Especially if the student is more interested in belt level than perfecting what he has been taught.
Sensei told me that the current belief is that it takes about 4 years of constant training for a student to perfect the Kihon katas. Those are the 5 fundamental katas for our system. Now when I started Pinan Shodan, Sensei was out for a surgery and after three weeks, I finally got my form corrected. It was the most subtle correction, but made a world of difference. I noticed immediately that my error was causing a tightening in my back. I relate this experience because a student could suffer a financial crisis and be unable to make it to the dojo and then begin relying wholly on that DVD. After a while, it becomes muscle memory and may be uncorrectable. Without an instructor being there, how can you be absolutely sure he isn't getting sloppy in his form and about to tear a meniscus, blow out a knee etc?