I'm tempted to call this novel nostalgic, for all of its debatable purpose and philosophy. Of course I wouldn't be the first to call it so, and part of the reason the jacket calls it "The controversial classic of military adventure" is the long history it has spawned of civilian readers being to some degree upset or suspicious of the almost fond way in which this future society of citizenship through military service is described; but still I had promised myself going in that I was going to try and find Heinlein's real philosophy behind the novel. As it turns out, I'm really feeling a lot of nostalgia and sentimentality from the book. Is that surprising? I think that even though you have characters like Mr. Dubois espousing the nature of the beauty of war and the enforcing of a moral system through punishment, sometimes Heinlein seems to be bringing out the very humanity he seems to find in such a system. The first time I really felt this was in the passage, starting on page 80, that describes Rico's experience overhearing Zim and Frankel discussing their strong feelings of anguish at the punishment they had just ordered for Hendrick, and in the process describing the very reasons they had been forced, out of an absolute loyalty to the system, to act against these feelings. Rico's response is his "unsettling feeling that I had been completely mistaken as to the nature of the world I was in." This isn't a passage the describes an undermining of the system, but rather a moment in which the main character experiences a epiphany about the significance and the actual (supposed?) underlying morality behind the value of adherence to a strict system for the good of the whole. Certainly Heinlein's career in the military (thanks Wikipedia) couldn't help but have informed this book, and certainly this is something that's been held against him by some critics. As a civilian, though, I'm as wary as the rest of you about such a rigid system like the one described by Heinlein (despite the humanity exhibited in private by some characters) in real life practice, and this book (though dated) certainly seems to try and nestle itself closely to the real world.
One classmate mentioned something about the tendency of Greek plays to include a character whose only purpose in the play was to expound to the audience about the philosophy the writer has attempted to embody in the play and his reasons for writing it to begin with. Interesting to see that method popping up again here, in the form of mister (colonel) Dubois. He doesn't just have one passage in which he seems to be lecturing not only Rico but the reader, but one of the most memorable surrounds page 118, at which point he begins to discuss mankind's cultivated "scientifically verifiable theory of morals." This is apparently the theory on which the entire society has been based, a society in which Citizenship (which means more than just suffrage) is awarded to those who voluntarily perform military service, and in which the MI are the most deserving, the most understanding of the morality they are upholding and the honor which they claim because of it. Everyone, the recruiter, Dubois, Rico, Zim, Ace is proud of the MI legacy and seems to belong to a club of understanding.
And yet there are the bugs. They are unafraid of defending themselves, they are willing to sacrifice all in the name of the whole. In many ways they embody the "noblest fate" described by Dubois, throwing themselves relentlessly into war and in front of their home. And yet humanity, for all its rigid belief of sacrifice for the whole, does not recognize this. Sure, they're bugs, but if they were evaluated by the same system as the soldiers evaluate themselves, wouldn't their be something that would have to be said for bug morality and bug civilization? Dubois mentions in passing that humanity is "even developing an exact method for extra-human relations." (118) Because humanity's method for inter-human relations is restricted to humans. Yes, here I can see it: the potential for problematic reading of Heinlein's universe as one in which different systems of evaluation for different beings goes unquestioned. How different to the beings have to be before they lose a place in the human system? The "Skinnies" aren't anything like the bugs. And yet humanity has put them in the same moral place: far away from itself.
I like the way you're thinking through the novel, and am particularly taken by your last paragraph and the implications it bears to understanding the relationships among these different groups in relationship to the exercise of colonial power...