Waste of money and it might actually work against you. Your VRM cooling heat sink relies on any inner case air flow to cool the VRM's, if you duct your H212 straight out the back, then that cuts out any airflow the VRM sink would have received, not to mention it also would block any airflow coming from fans on the side of your case (if you have any installed). If you really want to add a duct, I would suggest adding a fan+duct from the front 5.25" bays to the front of your H212+, that way it would draw in cool air across the sink.
Edit: another fan in pull on the backside of your H212+ would be a much better purchase.
Edit2: or even a 120 as exhaust in the rear.