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.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)