I agree with those who believe that "amateur" should be considered a different category from mainstream. I believe the term as originally used meant "mainstream media", implying something from TV or a "real" movie, but has gradually become far too broad a "catch-all category" in these forums, IMO. In fact, I wish the forums had distinct categories, or at least distinct "labels", for them, as it seems to me that there's a pretty big difference between amateurs just horsing around and a scene from a professionally produced TV show or movie.
But until there may be an "official" category for these things, perhaps people could just label them as "amateur" or "youtube vid" or something, rather than "mainstream"? Not that I really expect to see a consensus on this, if some people just may not be able to see the difference...