I watch video's on Youtube, are they really considered a valid reference when discussing/practicing Martial Arts?

In a nutshell, no. Video's are often rehearsed and of course staged to get an acceptable and usable final production. They are a tool to assist in training and practicing martial arts or any other discipline.

If I were asked to watch a video and then give my opinion on that persons ability in martial arts I would have to say, no way but I could give an opinion as to the overall production of the film much like I would a movie.

Also,

Recently I have read posting where the poster considers it necessary to prove oneself in production of youtube video's, etc. It came up when a comment refuted something posted and the poster shot back that unless you have video's from youtube, etc. along with other like materials your point was not considered valid, correct or even truthful.

Since when did producing video's become a serious validation or discredit of one's ability in marital arts or just about anything? I have seen a variety of video's on my system of practice that look really good but in essence are empty of any real meaning. Does this pretty and aesthetic form of production indicate proficiency in martial arts or is it just a production that looks really cool, good and pretty? 

Aesthetics do not make the martial arts even in sport. Often the most proficient martial artists have ordinary kata and often are not pretty. When a technique is pretty it is often because it violates all the fundamental principles of martial systems that make it effective - often, not exclusively.

I have seen some of the worst marital artist produce some pretty good kata DVD's and I have seen some ordinary video's from guys who I would not want to fight in sport, monkey dance and especially in the street.

If I present a comment and am asked for sources such as would be found in almost any analysis I would then present a bibliography which includes references to experience and conversations/seminars/practice with others who had or have the experience, background and knowledge to be an expert in the field but video's? Not a chance because even ordinarily video's are often staged and rehearsed.

Think about it, do you accept any video as proof that something is valid, it works and is good martial arts? Remember there is always "the rest of the story" behind those final scenes in the clip.