OR...
It keeps people from doing bad things like "I don't care if you blockade me because I'm completely self-sufficient!"
I know. I've done it, and it's awful RP. If we're trying to come up with even a reasonable facsimile of international politics, we've got to figure in natural resources. Otherwise, blockades become ineffective, imperialism can't be justified (which is why you should support the RS, Garth), and a large reason for creating alliances and blocs would have disappeared. It simply doesn't make sense.
Besides, it's not like this is one of those tracker-type maladies. It's very simple; you choose your resources, and that's all. It's not like you have to keep track of them by logging into a calculator every twenty minutes, and trading partners are easy to keep track of.