You can easily link to videos, just like linking to any other URLs.
I think Embedding audio/video would disrupt the flow of discussion. Add to that the fact that most embedded "video" is really embedded flash, and flash has a long history of security problems...
Some of my favorite forums on the internet don't even allow embedded images.
I think it's technically possible to create a BBcode tag to embed youtube videos, though.
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.