Beginners may learn alot from watching a video of their style, however there is a very real possibility of trying to learn a technique too soon. If one was to watch videos of ONLY the techniques required for their belt level(or equivalent), they could pick up a an understanding of a technique, BUT if the instructor in the video performs a technique differently than the instructor the student will test under, it may cause confusion. As most people willl get bored watching the basic white belt\yellow belt techniques of a curriculum, they may go forward, trying to learn and work on techniques from perhaps blue belt or brown belt syllabus. Problem being, without the proper foundation and experience in the techniques of the lower belts, the risk of getting it wrong, or causing injury to self/others is pretty high. To use an example from judo, uchimata is a spectactular throw, but it is not taught to white belts for a reason. Make sense?