Yes, well regulated and managed migration can be useful - and I think multiculturalism is more of a symptom than a policy. People will bring their cultures and adapt to the things that appeal to them - restrictive policies discourage integration.
Can't say anything about the US. In the UK, the topic is highly charged - and fuelled with a lot of misinformation. In principle, the regulations are surprisingly tough - enforcement though tends to be patchy and incoherent. Policy is mostly done on the hoof (whether from the left or right), and not evidence based.
Evidence based policies should be applied - not what looks 'tough on migration' in the media.
Fine. I live in one of the multicultural hot spots in the UK, and can't really complain about this aspect. I'm annoyed by bad infrastructure and poverty / inequality driven symptoms such as low quality housing, low level crime and public disorder etc. - I cannot see a cultural element in it though.
I think it's a no brainer that cultural exchange is the basis for societal progress - attempts at protectionism tend to not only be impractical, but counter productive. I think it's more about management than a strict 'yes' or 'no'.
Decades of policies based on fear of public perception - not evidence. People fear strange and different other people - and tend to blame them as soon as things (like the economy) start going wrong. On the other hand, I don't think you can shape a 'multicultural' policy either - I'm probably almost libertarian (I'll go and wash my mouth out in a minute): I think there is nothing wrong with letting people into your country in pursuit of improving their quality of life - in the long term it will create loyal and productive citizens. Open societies thrive better than ones that attempt to be closed. For that clear, fair and evidence based regulations are necessary.
To the OP: that's my opinion. To the others: flame away.