Should a discussion of political legitimacy be taboo? For the people of Christendom, in most times and places, I believe the answer is "yes". In a normal, settled, non-revolutionary society any discussion of this sort would be strictly limited to a small and responsible circle of scholars. To the extent that the "man on the street" dared to engage such a topic, he would rightly be considered dangerous, if not suspected of treason.
But what about Americans in 2011? Should the topic be off limits? There is, indeed, something unseemly and dangerous about it. Consider a happily married couple who make a habit of discussing, from time to time, all the various circumstances that might constitute legitimate grounds for divorce, annulment, or marital separation. They are only talking about the possibilities, you see: they have no intention of actually doing any of these things. But let me tell you a little secret: something about the conversation itself makes these possibilities more real than they were before. It's best for married people never to "go there", never to let the idea of divorce, annulment or separation enter their minds. It's a matter of training and disciplining the mind. The same restraint applies to many other things: e.g., those of us who are not involved in making legal distinctions should avoid even thinking about the finer nuances of where legitimate art ends and pornography begins. The exercise itself is dangerous; it lifts the veil. We know what the good is, so let us meditate on the good and nothing else.
So it is also with discussions of political legitimacy. To have this conversation at all is to put revolution "on the table".
Unfortunately, we Americans are in a bit of a pickle. Our own government - itself the product of a rebellion that it publicly celebrates - long ago opened the question about its own legitimacy and boldly invited the argument. Our government's legitimacy is solidified, not threatened, by a revolutionary spirit in the people. The day our government regards itself as having any legitimacy of its own is the day it ceases to be legitimate in the eyes of most Americans. Russell Kirk argued persuasively that the American revolution was not a revolution at all, but "a revolution prevented", or even a counter-revolution. While that may have been the prevailing sentiment in 1776, today the popular mind understands our nation's "founding principles" in purely revolutionary terms. The language and thought of revolution is simply the air we breathe: for this reason the better minds and hearts among us have, in my opinion, something of a responsibility to face the topic (and its many corollaries) head-on.
Still, most of us have no business attending every argument we're invited to. For the sake of peace and order, and barring particularly egregious circumstances (the determination of which forms the true heart of this controversy), every man should assume the legitimacy of his own government. That is, in fact, the actual praxis of the Catholic Church, which has such a preference for peace that it prefers to err on the side of legitimizing usurpers, even handing a crown to Napoleon himself. Which reminds me: I believe that someday, perhaps in the not too distant future, the question of political legitimacy will be made easier than any of us dreamed possible. In the meantime let us fight with all we have to make those "egregious circumstances" unthinkable.