If it's a Wrathgate type cinematic, you might not even know you're missing one. Also, it's better to be completely surprised and have it in game than to be spoon fed a video off Youtube. That's why Youtube video's are considered Spoilers, like with the fall of the Lich King. Although that video was kind of a let down.
This is why the game I'm most looking forward to is Deus Ex: HR, instead of anything from the increasingly bland and contrived Warcraft universe.