Yeah, I've heard that too, but never really bought the argument. Flat bars are associated with a more upright riding position which some people prefer for urban settings but there's no reason you can't set drops as high as flats. The other argument is that the wider bars give you better leverage. If your urban commute takes you over logs and through rock gardens, maybe that makes some sense but you can get pretty wide drop bars that flare out even farther at the ends if that's a concern.