There's this notion that multiculturalism prevents a nation from becoming united, but that idea rests on the false premise that multiple cultures cannot share a political culture. In other words, the idea that Mexican, South African, German, Native American, etc. cultures cannot coexist while their members are united under the American ideals embodied in the Constitution is not only a direct contradiction to the current United States, but also completely illogical.
I like living in a place where I can expose myself to many cultures. And to be honest, I think many immigrants appreciate American ideals more than many non-immigrants since the former tend to purposely come here to experience them whereas many in the latter group either take them for granted or try to turn this country something it isn't. Consequently, the idea that multiculturalism hurts a country doesn't really line up with my experiences so I support it.
"It ain't what they call you, it's what you answer to." - W. C. Fields