Here's what I see as options presented in most debates:
Pro (aka "Repeat after me: we are all individuals...")
This is usually peddled by the various religionistas, but also by some others who in particular should know better. Essentially, the proposal is that every single group who cares to do so, carves out whichever niche they seem they need, and in it practise whatever rules, rites, rituals, culture, and whatever else they deem defines them.
All this, of course, with more or less utter disregard to anybody else in society (bar gross - but not mild - breaches of legality). This is viewed as being OK mostly by virtue of generously granting everybody else the same right to do whatever they wish to do. As long as, of course, this does not somehow hurt "us" (for any applicable value of "us"), where it is (also of course) always assumed that "us" have somehow more right to be hurt by "them" than vice versa.
How any intelligent being can argue that such state of affairs is fair and beneficial to a multicultural society is anybody's guess. I regard this as utter silliness, and can't escape the image of an old saying where "they get along same as a bag full of horns". And that's essentially what you get in this system: a host of incompatible lifestyles all vying for equal space and place in society.
I am sorry to be the one to break it to you, but this just can't work.
Con (aka "Repeat after me: we are all 'insert name here'...")
This is, of course, the pet stance of all who are, in their heart of hearts, as much of a racist, religious fanatic, or whatever other label you think is appropriate. They keep shouting from the rooftops that everyone wanting to join in their party (as if it were a fun one, to start) needs to dress and behave the same as they are.
What these types (and they are no better and no worse than those described above) forget to notice is that it isn't their party any more, and most likely never really was. Modern Britain (and medieval one, too, for that matter) is no more homogeneously Christian (for any given value of Christian) than is modern Serbia (and especially not the medieval one!) a homogeneous kingdom of Orthodox Christians. Nor is, of course - albeit sadly - no country (yet) a paradise of atheism. And so on, and so forth.
The simple fact is, you just cannot take different people with different cultures (and especially different religions), and mould them into a ball of unrecognisably uniform dough. It's not possible, it's not right, and taken to it's ultimate consequence, it doesn't do anyone any good in any run, long, short, or medium. If you don't believe me, go re-read your history books. Such a ball of dough sooner or later (sooner) dries out and disintegrates into it's constituent parts and you end up with the horns in the bag again.
I am again sorry to be the one to break it to you, but this won't work either.
Now, it'd be very bad form, and you'd be well within your rights to slap me, if I just slagged off the two positions above without offering any alternative. If you were also possessed of a good dose of humour, you'd also demand I present any solution I may think I have as an apt metaphor.
Fret no more: I shall now do both...
What's more, I shall also build on the metaphors I already used, of horns and dough balls. With those in hand, and with a little bit of magic I borrowed from my religionista friends (and maybe a whiff of evolution thrown in for good measure) I am proud to give you... a hedgehog. Or, maybe better if you are more of a sea-child like me, a sea urchin.
Bingo (aka "sea urchin/hedgehog solution to world peace...")
Some among you may have already guessed where I am aiming with this. Some (hopefully not a lot), may have also started thinking I'll propose yet another wishy-washy peace on the playground sort of things. Finally, I am sure a vocal minority will insist I have done just that regardless of what I actually say. To them I say: stop reading now, go join someone else's tea party...
For the rest of you, here's my rationale:
It is an inescapable truth that we are all different. Individually, and as part of any group you care to draw a circle around and name something. This is as natural as it is necessary. After all, where would progress in any field come from if we were all exactly the same, behaved the same, and had all the same ideas and urges. Barring divine intervention - and you will excuse me, religionistas, but you may believe there is one, but there just isn't - from a uniform mass of individuals no new idea can ever come. So, even if possible, such an environment would be barrier to human betterment. And I hope we agree, human betterment is a "good thing"™.
On the other hand, it is equally unlikely that a random hodge-podge of different people and groups all pulling in different directions is going to achieve anything useful in any given period of time. It would be just like expecting a cloud of randomly darting Helium atoms to spontaneously move the balloon towards the lock, then form itself into a key and escape the birthday party. That'd be just a silly expectation, so I don't really know how anyone can think a society will benefit from such and arrangement. Not to mention that Helium atoms at least are not hostile to one another for no good reason, unlike some groups of people.
Add to this inability to facilitate progress the simple (and hopefully obvious) fact that neither a dough ball, not a random collection of horns can defend themselves if the need arises, and I have maybe given you reason to believe that neither can truly and successfully be applied to a human society. It may also become obvious that a human society needs to be organised in such a way as to make progress (aka human betterment) not only possible, but also probable, and at the same time be able to defend itself from whatever comes its way (sadly, it is still mostly other human societies that we need defending from, but that's another story altogether).
And this is where horns and balls of dough come together into a rather efficient system that can actually accomplish sensible goals, evolve and improve over time and generations, and also have a rather effective self-defence system. Both a hedgehog and a sea urchin have quite uniform cores of "dough" that represent most of their inner urchinness and hedgehogness, the bit that makes them tick and is responsible for any progress they may make in their lives. On top of that, quite literally, they are studded with horns they can employ as defence when needed. These horns, especially in a sea urchin, are quite free to act on their own as they best see fit, but still all in pursuit of a common goal, the goal set out by the ball of dough that is at the centre of things.
In a hedgehog or a sea urchin, this ball of dough is made out of all the meaty bits and pieces and other similar stuff an animal needs. In a human society, I submit that the central ball of dough must represent all that is common to all people, regardless of their culture and background, religious or otherwise. And if you want to really look into what these common things are, you will find that they are myriad, and that not really a lot is left once they are separated out of what looks like an incompatible human mess.
What is left, the differences, the horns that would otherwise be trying to run the show and break out of the dark bag where they rattle randomly against all the others, these can now be gainfully employed as a protection. However, by protection I don't necessarily mean protection against another human society (another hedgehog), but protection against the unknowns that the future has in store for all of us. Protection against running up a problem each one on its own would be ill prepared to tackle, but which can be solved by either concerted action, or a better suited neighbour.
But, and this is at the core of what I'm trying to say, it is the massive ball of common human ground that keeps all of this together and gives it purpose. And as humans we have much, much more common ground than we have differences any way you want to look at it. You may think your religion (or your atheism, for that matter) runs to the very core of your being, but if you are you are fooling yourself.
If you carefully separate out things in your life that are directly affected by what you see makes you different from your fellow human, and than have a look at what's left - and if you're honest to yourself in the process - I am sure you will find that what's left is such a massive part of what you are that the "differences" pale into insignificance. Which they shouldn't, because we all need them so we are as a society better prepared for what comes next (and no, by this I do not mean the Second Coming, or any other religious silliness).
So, let's press our common doughness into a tight ball and protect it with our differences as horns. It can only make us stronger. Oh, and now I see that a hedgehog is a better metaphor here: unlike a sea urchin, a hedgehog can open its soft underbelly and embrace new things into its core.
Therefore, at the end of this post, I give you...
A fledgling hedgehog society!
(and a 50p coin)