On the surface Dr Williams' remark seems a pretty daft thing to say, but giving it some thought it leaves me with a couple of questions.
In civil matters (divorce, property disputes, etc.) it may make some sense, as long as all parties agree. Then it becomes mainly a contractual thing (beware, IANAL). Still, let me emphasise may and some. I'm not qualified to go any further, so I'll leave it with a question mark: ?
The bigger question I find is that of what it means to be a 'multicultural', 'diverse' or 'plural' society?
Law is a different case, of course, but more generally if we want to be able to say we accept and value other cultures, are we kidding ourselves if we then go on and insist that cultural distinctives get left at the door (or the customs checkpoint)? How do we integrate and celebrate difference, rather than ending up with some sort of cultural lowest common denominator where differences are glossed over in case we find we disagree on something? (When did we forget that it's okay to disagree?)
I had a chat with a friend recently who helps out with a youth group in a church of a mainstream Protestant denomination. One of the young people who comes along is Roman Catholic with a big wide Republican streak. This young person asked if they could do something to commemorate the events of Bloody Sunday, as you'll surmise a controversial suggestion in that context. I don't know how that conversation went, but consider this: with all this talk of A Shared Future, how can we acknowledge and accept a shared past and a shared present?
Love him or hate him — I've many friends who'll be distressed when I say I have a soft spot for him — Rowan Williams has certainly got people talking.