I'll bet good money, however, that however many signs your <2yo son learns from watching a DVD, he would learn more from signing with you instead of passively watching the device. Which, for verbal speech, is exactly what the studies show.
1. I'm really not equipped to show him the multiple images of a coat or someone putting on a coat, or other children doing signs of coat. The video is. I don't know if this matters or not. It's certainly possible it doesn't.
2. I have to make dinner for us all at some point. As much as I'd love for my 15 month old to patiently engage in conversation with me while remaining a safe distance from the stovetop, it just doesn't happen. There are times when a parent's attention just cannot be 100%, or even 75% on their child. So yes, maybe he would learn EVERYTHING better if I were there to teach it to him, but the fact is that even the best parents can't be.
As for point 2, I don't disagree at all. Everything is a tradeoff, no parent can be perfect. Most kids turn out fine anyway. If you have to give your kids TV (my 3 year old gets about two hours a week, for instance) then do so and don't feel guilty. But don't try to justify it as educational; the science disagrees.