I think that speculative fiction is a very useful tool in examining philosophy, especially as it tries to explain human nature. In speculative fiction, the author takes people outside the normal parameters, by definition. In this way, we can dissect the human experience, and figure out more about the world around us. For example, the concept of time is not very commonly explored, except in speculative fiction, when it is commonly talked about in stories related to time travel. Because time becomes the basis of the entire story, it is inevitably explored in greater depth throughout the story, even if it is in speculation. The concept of mortality is most commonly addressed when dealing with immortality. Mind control brings up issues of what will and consciousness really are. There are so many issues dealt with in speculative fiction, that even if the author is using them as rhetoric, they still have great benefit to our understanding of philosophy.
Another benefit of speculative fiction is that it gets people out of their native environment. This is especially important when exploring the nature of people, because it allows people to distance themselves from social elements, that would otherwise be too close to view objectively. Take prejudice, for example. A person might have their own ideas of the social order between the races of the world, which would have an effect on their understanding of prejudice. However, if you have a story in which the conflict revolves around a group of aliens being discriminated against by other aliens because of their antenna length, it will give the person a much more objective viewpoint. From that viewpoint, it is easier to really observe the nature of prejudice.
So, I think that speculative fiction can serve as rhetoric, or as philosophy, or even both. It is a good point that, since an author gets to, in essence, choose who the reader has sympathy for, and who the reader doesn't, it is a very effective persuasive tool. However, I think that, as a whole, speculative fiction does a lot to explore areas of philosophy that other areas of literature tend to neglect.