Yes, metaphor allows people to access audiences that might be less than open-minded. However, let's get back to the Star Trek episode you mentioned. I could see a racist person living in the Deep South in the 1960s watching the black/white episode and saying, "I see what they're doing, but it's not really like that. Segregation is right because it's not just about skin color. There's a difference in culture." In other words, removing the moral from the context runs the risk (a very high risk, in fact) that the moral can be dismissed as inapplicable.

It would be much harder to deny the pernicious effects of racism if that person read (or watched) To Kill a Mockingbird. There's nowhere to hide. There's no way to justify the inapplicability of the situation. It IS the situation.

Also, I know you really, really like A Song of Ice and Fire and I'm not looking to open old disagreements, but I don't really see any deep messages or themes there. I think your predisposition to the series has elevated it a bit in your eyes. Yes, it talks about power. Yes, it's got a lot of intrigue. It's a lot like the HBO series "Rome" in that respect, and I think that's part of what drew HBO to it. I would still classify it as primarily a "diversion", however. A very good diversion (leagues better than Dan Brown's books or Terry Brooks or Goodkind), but still primarily a diversion.