I like to think that splitting the Great Artist into three categories will mean that Great Writers will not be required to be "Arty" writers, if you catch my drift. For example, judging by the Great Writers ability "Create Political Treatise", it sounds like GW's aren't only writers of fiction, but also political writers, debaters, philosophers and so on.
This could be a shortcut to including other historical Great People that doesn't easily fit into any of the original categories - for example, Dag Hammarskjöld is by some regarded as one of the greatest diplomats of all time, and he is also somewhat famous for his philosophical writings. Do you think he would classify as a Great Writer? Are there other people who would fit the label GW, if we extend it like this?