To all those claiming that multiculturalism has no downsides, I would like to point out that "equal time for creationism" sprung from and used multiculturalism; the notion that you can justify anything using religious freedom can and does lead to Bad Things being justified thus. AFAIK no real society is perfectly multicultural, but that's poor implementation; a bug, not a feature.

EDIT: I am in favour of all the Good Things that spring to mind when we hear "multiculturalism", and do not advocate the Bad Things associated with opposing it (ie a single monolithic and enforced culture.)