Comey needed the public hearing not for transparency per say, but it was to get intel to DS/Cabal/bad guys. In a way he tilted their hand, by challenging the subpoena, it means that his cohorts don't have enough intel to go to a plan B or C. They wanted an open book exam and the Professor just gave them the birdie!
When one is to be interrogated/questioned, the questioner generally does not ask questions he/she does not already have the answers to. He answered some questions in prior hearings, he has a chance to correct some errors or bury himself deeper. Open hearing would moat like allowed him to recieve some outside help via digital comms.