Why Exclude Audio from Instructional Video

Random rant from http://www.MartinKoss.com - Last time I checked; Video = Audio and Visual content... It always amazes me when I find a video on the web that includes no audio. Usually the 'screen capture' type instructional video include the 'instructor' actually speaking and explaining what s/he is doing. But, occasionally I come across instructional videos that have no audio - and I wonder why? Why waste the opportunity to really get your point across? If you don't use the audio to explain what you are doing, you might just as well use screen shots as still images. When a viewer watches a video - naturally they expect to get all the information without having to strain their eyes trying to work out what the instructor is clicking on... It becomes hard work. So: Turn on the microphone and don't be shy.