The big difference between countries like the USA, Australia, Singapore, Canada, etc., and most nation states in the world, is that the first group defines themselves as a country of such and such laws where individuals agree to such and such, while the second group defines themselves as a country of individuals of a distinct background and culture!
Most (all?) nation states can not integrate foreign people in such an amount as the melting-pot countries can, for the simple reason that they would lose their raison d'être!
Without having nation states as countries for one specific group, there is really no reason to have them at all, and we can all just become the United States of Earth, with total diversity and no special areas for each culture.
While such a world wouldn't be bad per se, I much more prefer a world where there is a difference between going to or living in Pakistan, Kenya, Russia, Iraq, Japan, China, Peru, France, Germany and so many others.
But of course, if I'm the only one who thinks this way, I guess I must just drop my idea of nation states, as I can't be expected to uphold this idea by solitary self. But to me, that would also mean that my Norwegian identity becomes worthless.
Well, I disagree, I suppose. I view mundane co-existence as preferable to romantic hostility. Perhaps it comes from the fact that I live in a nation where the "raison d'être" is formally divined as civic, and not ethnic, but I see no virtue in the anti-individualistic essentialism of the ethnic nationalist.
Nor, for that matter, do I see us devolving into the grey cultural sludge which you assume; it betrays a fervent belief in the inevitability of the monoculture, which is the very thing I reject. And not on idealistic grounds alone- just look at modern society when compared to a century previous, and witness the diversity which is so mundane as to be below comment. Aside from anything else, integrated minorities inevitably move closer to the natives than the reverse, and so, while we'll all be a little closer (as if this was a bad thing!), we're hardly going to end up as five hundred million square miles of American suburbia.
(And, for the record, Pakistan is a very poor example- it's a constructed, multi-ethnic state with no independent history before 1947. It is a civic nation and religious nation, not an ethnic one; it's apparent uniformity is the result of a strong nationalist feeling, a tendency for shared religion to override ethnic divisions, and Western ignorance.)
As I said multiculturalism comes in many forms. But it's fundamental ideas is that each ethno-cultural group should keep a separate identity, and the State should recognise and encourage that. Opponent of multiculturalism oppose that.
As I've been saying, the US is not multicultural at all. Being multi-ethnic does not mean being multicultural. The US is a melting pot, the different cultures are blended and newcomers are quickly assimilated to the dominant culture (with the arguable exception of the Southwest).
And, as I said, I think that you're conflating one segregationist form of multiculturalism with the idea in its most basic form, which I believe is inaccurate. Maintaing a distinct identity does not mean segregation;
I have a distinct ethnic identity as an Irish-Scot, but that does not mean I live sequestered in the Paddytown ghetto. It is possible to be different from everybody else without having to physically remove yourself from them.
Also, I would contest the assertion that American minorities have been "assimilated", as the term is traditionally concieved. While each obviously moved towards the established norm, but in doing so were able to both exert influence, and, in many cases, to maintain themselves, in certain ways, apart from it (it's not as if we can conflate the experiences of German and Chinese immigrants). If this were not the case, then every scrap of America would seem like England removed by six thousand miles, which is quite self-evidently not the case.
