Yeah, but when NGUI provides this class, you start using it everywhere but notice it actually works best for mobile games, while allowing this weird scrolling capability on PC without an option to turn it off.
Inheriting is a bad design to solve this. Of course I could have used my own class from the beginning, however I'm not alone on this project and I had to walk for half a day through years of GUI prefabs using it just to replace UIDragScrollViews by UICustomDragScrollViews.