I think it depends on three things. The learners background(karate, kali, kung fu), the quality of instruction in the video, and what techniques are being shown. Meaning if the learner has a good Karate background he will have an easier time learning new striking techniques from a video, but will have a harder time learning throws from a video judo without a grappling background. The quality of instruction varies greatly video to video. If they explain the dynamics and application of the techniques, then yes you can learn a lot from videos. Essentially if you have a good foundation you can add many things onto it as long as it's related to what you have experience in.
What Anarax says. Dvds can be a valuable tool, especially for those who have been dojo-trained for a long time.