Diversity is supposed to be massively important and in need of much protection, for example from the homogenising forces of globalisation: a Starbucks on every street, oh the horror! Diversity has associations with nice things like freedom (more choice), beauty (more variety, more forms), tolerance ('let a thousand flowers bloom'), and truth (more possibilities to explore mean more chance of hitting on the truth of the universe, etc). I suggest however that unless we are more specific about what what we mean by "diversity", talk about its value is essentially gibberish.
When we do try to define diversity more precisely, we are forced to acknowledge its subordinate and contingent value and place amongst the things that are really important, like freedom, beauty, and truth. Some understandings of diversity are actually quite noxious, but when presented in a fuzzy enough way are affirmed by lots of normally sensible people.
Diversity of what?
Like that other appealing word, "equality", "diversity" is rhetorically powerful, but meaningless on its own. If someone asks 'are you in favour of equality?' you won't know until you figure out what they mean. It could refer to any goal from equalising human income, health-care coverage, opportunity, the flourishing of non-human species, and any of a gamut of political positions. You have to ask 'Equality of what?' before you can know what what you're agreeing or disagreeing with. Similarly, diversity can be connected with almost any political philosophy from conservatism to socialism to libertarianism. So one cannot be for or against diversity per se, but only a specific understanding of diversity, for example maximising the conservation of many distinct human ways of life into the future.
A related concern is in setting one's preferred understanding of diversity into an evaluative framework. It is one thing to say one is in favour of say 'cultural diversity' (which of course need not be the same thing as 'conserving distinct cultures') but then you are probably also in favour of an end to hunger, war, and cancer. The question is how to integrate that concern into your ethical understanding of (evaluation) and approach to (choices) the world. This will include some analysis of how much you, or anyone else, could do to advance those values (ending hunger seems more achievable than cancer; both are somewhat beyond the capacities of lone individuals). But it is also important to consider, at least loosely, how your valuing of diversity should be coordinated with your other principles and values, like efficiency or respect for other people's freedom of choice.
One common understanding of diversity is defined over distinct objects (tokens) of a certain type, for example different human languages, different skin colours, different cultures, or - moving beyond the human level - species and ecosystems. Here diversity really means variety; and more variety is better. But why is that valuable? Some would respond that variety is intrinsically valuable (i.e. valuable in itself: no further reasons need or should be given), but I don't think we can let anyone get away with ending their answer there. Just because you claim something is intrinsically valuable doesn't mean you can't talk about that value, and its relationship with other values people tend to hold. That is important because every type - 'diversity-of-languages', say - needs to have a definition of what constitutes a token (a real language) as distinct from non-tokens (i.e. mere dialects). It is also necessary to distinguish the types in which it is important to have diversity from those where it is not (e.g. who supports a diversity-of-building codes in earthquake regions or a diversity-of-calorific-nutrition for new-born babies?). Both of these require some embedding in typical human concerns and perspectives.
The other concerns we have should affect how we think about diversity. For example, it is possible to argue for promoting biological diversity in the sense of conserving the variety of species that now exist, because they are beautiful, because they are irreplaceable, because we may be able to learn from them, etc. But we shouldn’t take the same perspective on human diversity as a responsibility to preserve existing variations – languages, values, religions, skin colours, etc - because humans have freedom.
Although it might seem that freedom would be closely associated with diversity since freedom is often thought of as choice, and more tokens must therefore mean more choice, in fact diversity is in clear opposition to freedom.
Firstly freedom in the liberal understanding is better understood in terms of the moral autonomy of the individual, from which the importance of choice follows. This means that individuals are entitled to their own valuational perspective in deciding what does or doesn’t matter to them, such as tradition, or diversity. Freedom does not only mean selecting from an approved menu, but having a say in the menu itself.
Secondly if people are able to choose what they want, their choices themselves will affect the shape of the choice set in the future. For example, there are around 5000 extant human languages, but only 200 have more than a few thousand speakers. The main point of a language, which is generally a lot of effort to acquire, is to communicate, and people will choose for languages which enable greater communication. Languages perceived as relatively less useful will be chosen less, making them ever less useful and less attractive until they die out.
Diversity conservationists implicitly expect that people will have a valuational perspective that conveniently suits their conservationist project, and that they will choose to stay the same, but they won’t, and they don’t. Although all those varieties may look very quaint to the detached observer, and may stimulate all sorts of insightful philosophical self-scrutiny in him, their conservation often requires sacrifice by the actual actors who are stuck in them!
Diversity in the human world must be subordinate to human freedom – it must serve the interests of people, and follow from the exercise of individual freedom. Diversity can be a valuable aid to freedom as moral autonomy. Being able to see the wide range of human cultural variations is not only awe-inspiring, but also helps open our eyes to the counterfactual possibilities: what we take for granted within our society we can now see can be done otherwise, and perhaps better. Maybe if Aristotle and Plato had been able to stomach looking at ways of living beyond Greece they wouldn't have endorsed infanticide as rational and moral.
Diversity is continually collapsing under the choices people make. It cannot be conserved with freedom, but it can be renewed. So, as Mill suggested, by all means let us encourage and support creative ‘experiments in living’ and try to learn from them what we can. But observers and actors must be equal – there can be no set of people – ‘philosopher anthropologists’ - who get to look out over and appreciate diversity’s charms while requiring another set of people live out their lives as guinea pigs. The point of doing experiments is to learn from them, and the point of freedom is that the people in social experiments should be able to learn, change, and choose as they see fit.