DeWitt will hit a HR now and then, unlike Theriot. He is not just a slap singles hitter. He was projected to show more power (a latter day Koyie Hill, so to speak, another Dodger prospect who went bust), but has never panned out.
He is not a great defender, but Theriot was a mediocre defender at best, so I can only judge DeWitt to be an improvement. Besides, he and Castro could make a nice tandem, as they get to work together on a daily basis. And DeWitt CAN play 3B. That may be important if Ramirez is either traded or moved to 1B. Of course, that leaves a gap at 2B again, but the Cubs still have Baker and Fontenot to pick up the <slack.