Multiculturalism just means more flavours of sexy
But the problem that people have with multiculturalism isn't about the present, and can't be found by looking at present conditions. It's about the past - an imagined past - and a future, usually a dreaded one.
Before Multiculturalism
The rational approach to questions of policy is guided by utilitarian considerations, cost-benefit analyses on a grand scale that are meant to capture all the effects that a given policy change may have on the welfare of individuals. Arguments concern the premises for your analysis, or its coherence of its model, or its predictive power, but no one, the recent Onion piece notwithstanding, challenges the utilitarian framework. To argue that policy should be formulated for a purpose other than benefiting the public - or at least some segment of the public, or to remove some injustice being done to the public - would be crazy. And not even bad crazy, but incomprehensible, weird crazy. Imagine trade policy being debated on non-utilitarian grounds. Or environmental policy. Or health policy. But when it comes to multiculturalism - or other issues deemed "cultural" - the frame breaks. Practical considerations are put aside, and support can be mobilized for policies that make no one better off and some people - the multis, to coin a term - much worse off. For committed liberals, libertarians, and conservatives that just care about money, that type of nativism is ugly and confusing, and tends to be chalked up to racism, homophobia, resentment or some generalized repugnant atavism. Once psychologized, the nativist reaction against multiculturalism can be dismissed as an irrational derangement. I think this is a mistake. This thing is deep, and it's smart.