While I have been waiting to resume my martial arts training, I have been watching a lot of videos on the internet. Additionally, I have been reading articles as well. These supplementary materials are great if:
1) You want an extra "boost" to take your skills to the next level
2) You can't access that particular martial art in your area (i.e. the nearest pankration school is 150 miles away).
3) You are injured/ill
4) You don't have the money to take your martial art of choice yet (in which case, studying such materials should be a reward for an eventful day of job hunting)
5) You just want to have fun
That being said, supplementary materials should NEVER replace hands-on instruction. This is because:
1) A DVD can't tell you whether or not you are doing the technique properly. A reputable instructor will stay on your back until you get it right.
2) A book can't show you how to adapt to deal with different types of opponents. This can only be learned through sparring and rolling on the mat with training partners.
3) Most internet sources (for example, Wikipedia and Youtube) don't have a strict vetting process. A reputable instructor will only hire teachers who are skilled in their craft. In contrast, anybody with a computer can post whatever they want on the internet and call it martial arts training.
Basically, supplementary materials are like protein shakes and energy bars. They can enhance your performance, but should not replace the chicken and pasta of hands-on training at a reputable school. You wouldn't tell your kids not to eat fruit because they had Flintstone vitamins in the morning, so you shouldn't tell yourself that you don't need to take martial arts lessons because you watched Jon Jones' highlights on Youtube. A lot of people will tell themselves that they only need to learn from books, DVDs, etc. because they are afraid of getting injured in a real training environment. While injuries do happen, these are pretty rare in a reputable school. Also, there are an increasing number of "women only" martial arts classes for women who are intimidated by the prospect of fighting men, or who simply don't want to get hit on. So while there is nothing wrong with using supplementary materials to learn a few extra tricks, make sure that you are using them for what they were intended for: to complement hands-on training.