The sharing of information in regard to as fragile a meme as aikido can only be good.
Obscurity, rather than questionable quality, is what hurts many martial arts (if the goal is for the art to survive).
I think Lyle is correct. There are some styles of Aikido that are severely lacking by way of exposure and availability of information and media, video or otherwise. Sites such as Youtube present a small sampling of what is actually happening in dojos with all levels of instructors globally and may make available things that one may never become exposed to unless one were to travel to Japan or if one were to wait for their instructor to bring out a DVD.
As far as questionable quality goes, well it depends on what one qualifies as quality - across the multitude of Aikido styles there is no common measure of quality so there will always be misjudgments in that regard. Most people only know what looks good imho.
As far as decreased DVD sales go I don't see Youtube being as big a problem as peer to peer file sharing actually. When I can get just the video on Youtube I can get the entire original DVD via a torrent - there is no comparison really. If one is serious about ones training and supporting ones style or instructors then one buys the DVD imho.
Like anything else its utility and benefit has a lot to do with the person using it. There are obvious limitations placed on human intelligence, but none whatsoever on our stupidity.