The definition of a society must be a group of people who share common core moral values, who accept a common legal system and, create their own common culture. A society's common culture is the total sum of their own belief systems and the traditions and creative products that emanate from them. For that reason, the idea of a multi-cultural society is in deed not only flawed but a contradiction in terms.
Kit appears to have commented on the premise as it was presented by Theo rather than the legitimacy of the conclusion. Anyone, who travelled in Arab Lands, will know that, for example, a Sunny, Saudi Arabian society will NEVER, at least in the foreseeable future be part of a multicultural society where they would have to share beliefs, values and legal systems different from their own.
It is only in Western Europe, particularly in Great Britain, where this nice sounding idea of a multicultural society gains political significance, such an experiment is given a chance to fail.
Georgebundle