Tanaka Reina started working as a part of Morning Musume when she was 13 years old, she has been around for 10 years. I think that its a must to stream her graduation live on Youtube. Lots of fans around the world would want to watch her graduation, since it would be her last. And I'm sure that if it IS streamed live, many fans would be more than thankful.
And it will also help those who can't afford going over to Japan for the concert, I believe streaming live will also help Morning Musume gain a lot of fans, similarly with the rest of Hello! Project.