I think you are underestimating the inherent laziness of the human race - a youtube link is something you click and watch.
A demo is something you need download, possibly extract, put into a correct folder, run the game and launch.
It may not be much, but it's a psychological barrier that can mean the difference between watching it and thinking "I'll get to it later." never to return to it. Hell, this happens to me even with youtube links, so demos would have even less chance =)
That said, making it possible to record demos would be a great feature. Don't let this naysaing make you think that I wouldn't appreciate it.