TEV asks (in a comment below) whether a distinction can be made between the relevant and the merely "topical" in literature. I do acknowledge the difference, and would even admit that sometimes even "topical" fiction can transcend its topicality to be relevant both to contemporaneous and to subsequent readers--The Grapes of Wrath, perhaps. Clearly Steinbeck's novel engaged its readers on a level much deeper than the merely topical, and to some extent continues to do so--except when it is considered primarily as reflective of "social history," the death chamber of literature.
But Steinbeck did not, at least in my view, achieve this feat by striving to be "relevant" first and foremost. Instead, he effectively carried out his relatively simple idea--a family of Okies on the road--to its logical culmination. The story he tells proceeds at least as much as a skillful working out of this literary idea as anything else, and thus it is still able to provide some readers with a satisfying reading experience. If it didn't, it would now be worthwhile only as social history. That it also engages with larger political and historical issues is, in literary terms, a bonus. (That is, if getting your politics from literature is a motive for reading it.)
Stone's Dog Soldiers does not rise to the literary occasion in this sense. It is, at least to this reader, almost purely an attempt to "comment" on the Vietnam War and its effect on American culture. It doesn't have the same mythic resonance as a novel like The Grapes of Wrath and doesn't provide many other readerly pleasures, as I tried to point out in my initial post. And when it's all said and done, who really cares about the "commentary," no matter how relevant, supplied by a novelist? We ought to turn to novels and poems because their authors know how to create good ones, not because they have extra-literary "ideas." (Although some writers undeniably do.)