The system being the way it is helps make sure that less innocent people get railroaded into getting convicted for things they didn't do. While in this case its a slow process with an obvious conclusion, in a reality where the police arrested the wrong guy, a trial like this would possibly exonerate them.
Albiet, in a world where Brooks was arrested and innocent, his being a stupid cunt would have resulted in him being convicted anyway, but that's beyond the point. Despite knowing he's for sure getting convicted, he's still not going to just plea guilty