I think Heinlein actually has a very interesting point. To wield the power of the government (which is what voting is), it is important to be able to act selflessly. If someone can't do that, even for a couple of years of their life, why should they be able to wield that power over others? The universal franchise is not a religious dogma, it's good to ask these questions and think about whether our society could be better if we organized it differently. Unfortunately, a lot of people completely missed the point and just rounded it off to "Heinlein thinks the military should run society", which isn't at all true.