In a keepers case it is incredibly useful. We are specifically talking how good a keeper is in claiming the ball. A team might concede at set pieces for reasons not to do with the keeper. For example Guzan is a beast in the air, but Villa are terrible at defending set pieces. This is because Villa's defensive organisation pretty much doesn't exist.
Yeah, but aerial duels also aren't happening in the vacuum, the defenders still have a job to do when goalkeeper comes out to collect a cross. I'd imagine DDG success rate in the air goes up when you can field Vidic who can bully the attackers from trying to challenge the ball.