The common good is a formal concept, because it is defined in terms of what it should engender (a peaceful and efficient commonwealth) but not in terms of how to reach that goal: appeal to the common good are thus similar to appeal to natural law, as no one knows what is a natural law or a natural right but everyone thinks it is great to use the term. From a rethorical point of view it is obviously an advantage to be in favor of the common good because the alternative is to be against the common good or in favor of a common evil.
Jokes apart, the idea of common good has no substantive content as we normally have blurry ideas about it. First of all, there is a logically insurmountable problem in defining what is good, as there are no sources of value judgements but individual subjective preferences. Is it good to make everybody equal from a material point of view; is it bad to kill all redheads? There is no rational reason to know. Even though we knew what is good, we may not have the cognitive skills or the information required to know how to reach it. This is particularly true in a complex society, as it is a novelty in human history, and our instincts are not used to living in it, our main moral systems have evolved when there was nothing of comparable complexity, and our reason and our knowledge are extremely limited in their capacity of understanding the details of the outside world. This is typically austrian problem: if the world has "complexity reducing institutions", such as prices or customs or ideologies, that enable us to live in society even if we don’t understand society, and if these institutions are necessary to make a complex world possible, otherwise society would be limited by our cognitive skills and thus it couldn’t be based on more than a handful of interacting individuals, then we cannot know the details we need to decide what the common good is.
The common good is an abstract concept which reminds us that living in society is beneficial, and so we can’t do without such an idea (Jouvenel), as the natural consequence of saying that no common good exists would be to suggest that social interaction is harmful for individual human beings, and this would be as catastrophic as the ideologies of conflict in modern thought: conflicts of classes, conflicts of races, conflicts of nations, conflicts of civilizations. All these ideologies tend to deny that society works because it is better for almost everybody to live in society than to live alone.
Lacking a precise substantive content, however, the idea is at risk of being exploited for propagandistic reasons, and in fact more often than not the common good is considered synonim of collective choice, democratic choice, political choice. Gemeinnutz geht vor Eigennutz, said the German nationalists, and our modern political thought is based on the dogma that collective choices are the natural place to determine the common good. This is absolutely unliberal, unlibertarian, totalitarian and authoritarian. Locke didn’t have this in mind when he considered politics to be a limited chartered social body necessary to circumvent a limited number of problems inherent in the theoretical anarchist natural state of the world, whereas on the contrary Rousseau, the father of totalitarian thought, thought that there was no such a thing as a society without an omnipotent political assembly imposing order on it. This is the conceptual mistake at the root of the whole contemporary western ideology: that society is possible only because a political assembly declares its existence. Governments are no more entitled to fill the concept of common good with content than any individual human being, as it is at least theoretically possible, and I believe it is true, that common good requires more or less strict limitations on political power.