DCI Sullivan

What was the point of having DCI Sullivan? He doesn't seem to have any DCI duties. From time to time, he chats with Ashurst and Scribbins, and tells them to solve the case. He is not necessary. The female DCI in season 2, though irritating as hell, was a better DCI than Sullivan. And why did they make the Scribbins character so dumb. Anyone that clueless would never get a job as a detective sergeant.