The video is an option for people - to watch if they want and not if they don't. I agree registering may seem obvious to do but not for everyone - especially the email activation bit.
It was posted to YouTube as that was the easiest way for me to upload the video so it could be embedded here by haggisv. I also wouldn't call it advertising the forum - more raising awareness. If it was advertising I'd have included loads of great reasons to join and not simply "how to register".
I also think everything is relative. It's easy to do something if you know how. For example, do people know you can use the control panel to remove signatures and aviators from the view you see so the webpage is much "cleaner" and less cluttered in some peoples opinion. Or that at the top of each thread is an "email friend" option which is a very easy and fun way to send details of a great thread to someone who may not be a member - try sending yourself one. Or that there is a print thread option at the top of the each thread which if you click on some might some people might actually find easier to read than this threads view.
So many different things out there which some people might not know about. Unless you share it and the videos are one way, people may never know about them. For others yes, obviously it will seem obvious - but that will always be the case when you know the answer - as haggisv has pointed out.
