I asked this same question recently when we were establishing a standard category structure for locations to be used in the one name studies project. The thought behind a surname category was to index any family names that lived in a particular area, but to my way of thinking, this is already accomplished when the location category is added to a profile, automatically populating the location category page with names.
In my opinion, a separate category for Surnames is not necessary and could be removed, but if other members are finding them useful, then that's what is important.
ONS project leaders are finding great use in the ability to organize family lines by location and I believe it is helpful for people to visit a location category and see that there are name studies available for specific surnames. As more and more leaders populate their ONS projects with location categories, I would think that these categories would trump a surname category.