In a discussion with a friend the other day, he seemed to state that an editor has no business helping an author bring up sensitive social topics in ways that will not alienate a lot of readers. If I understood him correctly, he holds that it is the artist's job to do what she needs to, regardless of the consequences. What role an evaluative editor should play in this scenario he didn't make clear.
Perhaps he believes that evaluative editors get in the way of the writer's true articulation in ways that copyeditors or proofreaders do not. He may feel that artistic creation should be left wholly individual.
I don't agree with this. My sense is that in one way or another almost all writing, including great writing, is collaborative. For instance, I think Shakespeare "wrote" his plays, but it seems reasonable to assume he got a lot of help from his fellow players in The King's Men.
When editors and writers collaborate on discussing social issues, it is the editor's job, I believe, to point out the possible consequences of certain choices. To do so, editors need to keep on top of current sensibilities and sensitivities in the social sphere. They need to negotiate between these public standards and a writer's individual artistic impulse.
On the one hand, neither editors nor writers want to strip a work of its uniqueness. Who would want to publish or review such a bland work? On the other hand, if the work unnecessarily steps on some toes, it will run into trouble in a variety of ways—from finding a publisher to getting reviewed.
Writing is an irreducibly social act, with many people involved other than the person listed as an author. These include readers, editors at all sorts of levels, publishers, and reviewers.
Another such group is other writers. Each individual writer thinks about or approaches an issue in a different manner, thereby providing their colleagues with openings for alternative perspectives and angles. In this way, reading precedes writing.
Certainly, editors should not tell writers what to do. But they need to make writers fully cognizant of how certain choices may affect a large proportion of potential readers.