Split-screen multiplayer support doesn't add much to Dirt 2. Your opposition is more believable and some of the challenges are more fun, but the visuals get even worse. And the sound effects--which are at least inoffensive during solo play--crescendo into a confusing mess when they're playing for three or four drivers simultaneously.
The dozen or so licensed songs on the soundtrack are a good fit at least. When you add an infrequent but irritating bug that causes your car to stop and respawn when you clearly haven't strayed out of bounds to this already undesirable mix, you're left with a racing game that's definitely best avoided.