That seems like a distinction without a difference. The fact that a good many people fail to vote does not mean that the RIGHT to vote is not one of the perquisites of "citizenship".
Also, remember that Heinlein wrote STARSHIP TROOPERS for a juvenile audience- the details of libertarian philosophy were incidental to the action- adventure story. This particular scheme never struck me as being well thought-out, anyway- and I read this book when I was 12 years old!
The distinction, to my mind, if we're going to dice words (which I have a bad tendency to do) is that you can be a citizen insofar as you receive certain protections from the State and are attached to it in various ways, without necessarily being able to vote. It is sort of like having a chocolate cake without any frosting: good, but not as good as it ought to be.
On a practical level, I agree with those who say that restricting enfranchisement in such a manner is against the intent of the Founding Fathers. I don't agree with the idea. However, there *are* certain positive aspects to doing such a thing; sadly, they would be greatly overshadowed by the injustice inherent in such a system.
Tuor