Multiculturalism is promoted from the top ( policy makers in Washington ), it is one of their tools to weaken national identity and to promote division and conflict between the different ethnic groups. This, of course, is a great distraction from other more important issues like: Who is taxing you to death?, who is curtailing your constitutional rights?, who is imposing more controls on everything you do? and .... you get the idea.
Illegal inmigrants do not come here because they love breaking inmigration laws, they come because their alternatives are worse. Think about it, lets say you are a poor mexican trying to decide your future (and your family):
staying in Mexico: -starve slowly to death
-get killed by drug dealers
-get killed by cops
-prosper by selling your soul and working for or with the
corrupt goverment
So, given those choices, a big percentage of desperate mexicans choose to come to the US and give it a try, despite the many dangers.
So you say: Why don't they go back and fix their own country? Well, they have tried many times in the past 100 years, but every time it looks like they might do it, the powerful hand of Uncle Sam intervenes to save the status quo.
As you can see, it is a complex situation in which the general population of the US and the millions of illegal inmigrants are nothing but pawns in a very big chess match.
I agree with most here, that the cultural identity of the US should be preserved, and the most basic glue is the English language.
to be continued...