I know Patrick Chung has been in the doghouse to most people ever since the first Buffalo game this season, but Pat is the best cover Safety (asides from McCourty) we have. Tavon Wilson may have a better nose for the ball, but he's not nearly as instinctual as Pat and he gets beat on a double move too easily. Gregory is turrible, he's the worst tackler on a team full of bad tacklers, he takes bad angles, he's no good against the run and he gets sucked into a PA way too easily.
Patrick, on the other hand, is the best run defender on the team asides form Spikes and Wilfork. With McCourty out roaming the field, Belichick is free to send Pat on a blitz. He can do what he does best and play closer to the line, he's the best tackler on the secondary.
BTW, statistically, Chung is allowing nearly 53% of the passes thrown his way for completion, but Gregory and Wilson are allowing close to 67%.