Growl still has more uses than the simple notification paradigm being ported from iOS. I remember using growl tunes to display song titles music video style. Similarly I used different notification themes for different applications. Some notifications never left the screen and didn't have to be invoked in a separate shelf. Apple's notifications also won't display when AirPlay mirroring. These are all minor cases where someone might get more use out of growl (the old one anyway) than NC. Since they arent trying to compete to be everyone's solution, they can still appeal to those fringe cases.