Simple. If you want a service oriented economy, then diversity is a strength (think Silicon Valley or Hollywood - if it was unicultural, they wouldn't be as successful. Hollywood has had great success with Jewish, Black, Asian, Latino and other cultures represented in it. If they insisted only on WASPs, then they wouldn't have been an national or international success. Ditto for the Silicon Valley).

If you want to be purely a manufacturing country (think China or Japan) then multiculturalism is a weakness. Come to think of it, even in the military sphere, multicultural countries like the USA, Russia, the British EMpire, etc have been more successful than uni-cultural ones like Germany, Japan, China, etc