He wrote The Doctors Wife, which was brilliant, and Nightmare in Silver, which sucked. A 50% success rate isnt really representative of this horror-fantasy master. Its entirely possible that he fell into the Clara-shaped void of personality and had nothing to work with on his second episode, which would explain why the Doctor spends most of it talking to himself. He still deserves another episode to prove himself as the mastermind he is, though. He already said he's desperate to write for Capaldi, which is a bit like the Queen saying she wants to come to your dinner party. Refusing him would be like telling her in response that she's got a fat face. The hype created over his episodes boosted the series popularity and sucked in new fandoms, so the BBC should know to get him back by now, and to subsequently lock him in a cupboard to keep him.