I'm not sure you're seriously looking for an explanation, given how "bastardizerd society" makes it sound like you think you know already. Still...

I think a good working definition of multiculturalism is a "federation of cultures". Just as the individual colonies and territories of the original US didn't have to subsume themselves in a monolithhic national government, America doesn't require immigrants to shed their culture and adopt a new one "off the rack". We require immigrants only to fit in, to find a way to add their culture harmoniously to the mix. The result is very much greater than the sum of its parts. Think "hybrid vigor".

The idea of multiculturalism is not new. In this month's Atlantic there's a reprint of an essay from 1916 on the subject. The magazine requires subscriiption, but not surprisingly for such an old piece, I found a copy in a university library.

No reverberatory effect of the great war has caused American public opinion more solicitude than the failure of the "melting-pot." The discovery of diverse nationalistic feelings among our great alien population his come to most people as an intense shock. It has brought out the unpleasant inconsistencies of our traditional beliefs. We have had to watch hard-hearted old Brahmins virtuously indignant at the spectacle of the immigrant refusing to be melted, while they jeer at patriots like Mary Antin who write about our "forefathers." We have had to listen to publicists who express themselves as stunned by the evidence of vigorous traditionalistic and cultural movements in this country among Germans, Scandinavians, Bohemians and Poles, while in the same breath they insist that the alien shall be forcibly assimilated to that Anglo-Saxon tradition which they unquestionably label "American."

...

The foreign cultures have not been melted down or run together, made into some homogeneous Americanism, but have remained distinct but cooperating to the greater glory and benefit, not only of themselves but of all the native "Americanism" around them.

What we emphatically do not want is that these distinctive qualities should be washed out into a tasteless, colorless fluid of uniformity....

The failure of the melting-pot, far from closing the great American democratic experiment, means that it has only just begun. Whatever American nationalism turns out to be, we see already that it will have color richer and more exciting than our ideal has hitherto encompassed. In a world which has dreamed of internationalism, we find that we have all unawares been building up the first international nation. The voices which have cried for a tight and jealous nationalism of the European pattern are failing. From that ideal, however valiantly and disinterestedly it has been set for us, time and tendency have moved us further and further away. What we have achieved has been rather a cosmopolitan federation of national colonies, of foreign cultures, from whom the sting of devastating competition has been removed. America is already the world-federation in miniature, the continent where for the first time in history has been achieved that miracle of hope, the peaceful living side by side, with character substantially preserved, of the most heterogeneous peoples under the sun. Nowhere else has such contiguity been anything but the breeder of misery. Here, notwithstanding our tragic failures of adjustment, the outlines are already too clear not to give us a new vision and a new-orientation of the American mind in the world.
...
America is coming to be, not a nationality but a transnationality, a weaving back and forth, with the other lands, of many threads of all sizes and colors. Any movement which attempts to thwart this weaving, or to dye the fabric any one color, or disentangle the threads of the strands, is false to this cosmopolitan vision.
...
(T)he attempt to weave a wholly novel international nation out of our chaotic America will liberate and harmonize the creative power of all these peoples and give them the new spiritual citizenship, as so many individuals have already been given, of a world.

Randolph Bourne, "Trans-National America", from Atlantic Monthly, 118 (July 1916), 86-97, archived at Swarthmore College

One thing that stands out in that essay is the clear identification of "Anglo-Saxonism", rather than some hypothetical uniquely American culture, as what's being defended. That's a blind spot that you and I, Mr. Chandler*, need to be on guard against--as Anglo-Saxons we tend to be as blind to the nature of our culture as a fish is to the ocean.

The phrase "we have all unawares been building up the first international nation" is intriguing in light of the expansionist course set by Bush. If it's our ambition to build a global culture modeled on our own, then it's interesting to look at how the "American Experiment" might scale up to provide a template for such a global culture. I think of it as a thought experiment that sheds some light on which view of multiculturalism is the stronger one by seeing how far the concept can be scaled.

In your conception, if America wants to found a global culture, we'll have to elminate all other cultures and all other languages, and impose our own. We'll have to make English the official "World Language" and somehow force everyone to use it. The outcome would be "global Americans" instead of "Terrans".

I think my view of multiculturalism would be a much more viable template for a world culture. As in America now, a practical underpinning of laws and institutions would regulate the business of this culture--commerce, crime, rights and prohibitions, a common currency, etc.--while disturbing as little as possible each local culture. Concern over a common language has become rather archaic in light of the availability of automatic translation systems, and I see it as a moot point.

Which approach to a global culture stands a snowball's chance in hell of succeeding? And doesn't that tell you something about how well each approach works in America?

(* -- Said the Shepherd to the Chandler: "You light the way, and I'll round up the stragglers.")