I used to think this, but I'm not as sold on it anymore. There's no way I'd say he's good defensively, but simply looking at his CORSI/Fenwick is only half the picture. His offensive zone start % is fairly low at 45.8%, which makes it more difficult to have good possession stats. The same goes for +/-, where he's -4, above Edler, Stecher, Del Zotto, and Pouliot.
Again, I'm not saying he's good defensively, but I wouldn't be so quick to say he's one of the worst defensive defensemen in the NHL.