Multiculturalism is much better for the economy then assimilation is. Multiculturalism provides a lot of diversity in products for sale and what people buy, allowing the economy to be sustained from a lot of different things. Assimilation will only hinder the economy because it will cut down on jobs and make the economy more frail.
In purely economic terms, in the modern global economy a country must rely on constant innovation by its workforce. There are specific things that can be done to specifically foster an environment in which innovation can thrive. One of these is intentional creation of dissonance, divergence, and diversity -- all traits that thrive in a multicultural environment.