Multiculturalism can mean many things. It is unappealing to think that a country must have its institutions purged of their cultural inheritance to make newcomers feel more welcome. It is pointless to hope that all subcultures will have equal status and authority.
But the idea of a political nationality distinct from an ethnic or cultural nationality is a different proposition. It is true that such a thing will inevitably involve a lot of unedifying ethnic brokerage. Communication and trust will be more difficult than in more homogenous societies -- there is a reason that efficient welfare states are ethnically homgenous And there always lurks the nightmare of violent breakdown -- one has to trust pretty firmly in habits of civilization to make it work.
On the other hand, the very fact that ethnic nationalities are different creates a potential for gains from trade, and for breaking up the kinds of political cartels Mancur Olson went on about. The very fact that one's loyalty to the state cuts against one's pre-political ethnic affinities may make it harder to romanticize it, reducing the risk of tyranny.
This kind of liberal multiculturalism isn't just a wet late twentieth century invention. Here is Cartier's speech on February 7, 1865 to the Legislative Assembly of the Province of Canada:
Now, when we were united together, if union were attained, we would form a political nationality with which neither national origin, nor the religion of any individual, would interfere. It was lamented by some that we had this diversity of races, and hopes were expressed that this distinctive feature would cease. The idea of unity of races was utopian -- it was impossible. Distinctions of this kind would always exist. Dissimilarity, in fact, appeared to be the order of the physical world and of the moral world, as well as the political world. But with regard to the objection based on this fact, to the effect that a great nation could not be formed because Lower Canada was in great part French and Catholic, and Upper Canada was British and Protestant, and the Lower Provinces were mixed, it was futile and worthless in the extreme. Look, for instance, at the United Kingdom, inhabited as it was by three great races. (Hear, hear.) Had the diversity of race impeded the glory, the progress, the wealth of England? Had they not rather each contributed their share to the greatness of the Empire? Of the glories of the senate, the field, and the ocean, of the successes of trade and commerce, how much was contributed by the combined talents, energy and courage of the three races together? (Cheers.) In our own Federation we should have Catholic and Protestant, English, French, Irish and Scotch, and each by his efforts and his success would increase the prosperity and glory of the new Confederacy. (Hear, hear) [I view] the diversity of races in British North America in this way: we were of different races, not for the purpose of warring against each other, but in order to compete and emulate for the general welfare. (Cheers) We could not do away with the distinctions of race. We could not legislate for the disappearance of the French Canadians from American soil, but British and French Canadians alike could appreciate and understand their position relative to each other.
Image of George-Étienne Cartier property of the Manitoba Archives