The frogs kicked over a hornet's nest!

[The Grand Nudger]

Serve your country then become a citizen.

While I'd have to confess a little bit of sympathy for the notion myself (and of course I would..eh, lol?), we'd have to consider that a citizenry that was entirely comprised of soldiers and former soldiers would lead inexorably to a more militarized state. Soldiers aren't made by gentle suggestion or rational argumentation, but by deep rooted subversion of what we might call our gentler instincts. This isn't to say that military indoctrination turns us into automatons, it's simply the acknowledgement that it does have -some effect- on how we view ourselves, others, and what is or is not acceptable or even desirable when it comes to the resolution of any given issue. A conversation that we currently have (regarding the appropriate level of militarization) would be altered in a fundamental way if the only voice which mattered (and enjoyed full protection of law and full enjoyment of rights) were those of soldiers and former soldiers (though interpretations of just what serving your country meant in his novel changed as criticism was levied). It's the very definition of a two class system.

I don't think that Heinlein made a very good case in his novel (which was a vehicle for a political ideology that most americans would likely find incredibly disturbing). The movie that most are familiar with was actually an -intentional- spoof of the novel. For an opposing take (if you enjoyed Heinlein - I certainly enjoyed the book even if I don't completely agree with the ideology therein) try "The Forever War" by Joe Haldeman. Two very different authors with very different takes on life outside of their novels. Heinlein served in the Navy in the 30's and was a civilian by the time of WW2, Haldeman was a combat engineer in Vietnam, awarded the Purple Heart. Knowing only that about the two authors the differences in their narratives (and the ideologies contained therein) is illuminated immensely. They're both models of the same story, btw. Following a fresh recruit through his entry into service, assumption of a leadership position, and ending with the impending terminus of conflict and therefore reflection.