especially as it relates to immigration--some quick notes.
Unwritten custom has the force of law, and we should remember the power and normative nature of a society's
mores, even if we have lost the rationale behind much of them. We are dependent upon such unwritten customs to provide a guide for our action, even though we may be ignorant of them much of the time since they generally lack the coercive aspect that we associate with law. The common good is the good life of a community, and this is dependent upon law, both written and unwritten. One cannot live well with others if the rules governing life are in fluctuation or are being replaced haphazardly.
This would appear to be the Thomistic view, even if the authorship of
On Kingship is disputed by some. The author of
On Kingship (who I take to be St. Thomas) agrees with Aristotle that allowing too many foreigners into a political community can be dangerous, in so far as they bring different customs with them and thus be an obstacle to the moral formation of the citizens of that community.
Contrary to the dream of liberals, rules of 'procedural justice' (presumably accessible to all through pure reason) and respect for "fundamental human rights" are insufficient as guides to the good life, since they do not provide all of the norms associated with justice, both general and particular.
A multiculturalism that is tied up with moral relativism is not only bad in theory, but in practice as well, as it goes beyond affecting the curriculum of educational institutions but destroys the fabric of social life through its effects on public policy and immigration laws. Because important customs can have the force of law, a rapid change of
mores may not only destroy the native culture but diminish the respect for law as well. Hence multiculturalism, even if it is not linked to moral relativism, can cause it.
The question of how different the culture of various immigrant groups are from "American" culture needs to be answered. Because of the importance of cultural affinity, 'preferential treatment' towards members of certain groups may not only be licit, but necessary for the common good, if immigration is to be allowed at all. The other point that should be made is that because of the destructive effects of culture associated with industrialization, excessive mobility (and everything else that is associated with the "modern America") there may not be much native culture left to be preserved or protected in the major metropolitan areas of the United States. However, such a culture may still remain in some areas, and we should commend those who seek to protect that culture in the interest of the common good, rather than criticizing them as being bigoted or narrow-minded (assuming of course that the culture is morally praiseworthy in itself).