The way you are using the word "alliance" is extremely broad. Different groupings require different levels of solidarity. For two people to work together in political activism, they need high levels of solidarity and aligned goals, otherwise they will be working at cross-purposes rather than co-operating - hence "weird" alliances are difficult. For two people to shop at the same store, they barely need any alliance at all - just don't get into fights in the aisles! Vegans and body-builders who both shop at Holland & Barrett are absolutely not entering into an implicit bargain with each other, and if they use the products for different purposes, it doesn't disrupt the others' use. So "weird" alliances are easy - and over time, they dissolve the lines that blur the groups. I very much doubt whether vegans and body-builders do metaphorically "hold their noses as they put their cash down" - when they go shopping, they are just customers.
This, of course, is why the free market and the commercial culture are the great solvents of clannishness and bigotry - they facilitate these "weird" alliances by providing a common set of rules that allow everyone to pursue their ends without disrupting the others. As Voltaire wrote as long ago as 1733:
Go into the London Stock Exchange - a more respectable place than many a court - and you will see representatives from all nations gathered together for the utility of men. Here Jew, Mohammedan and Christian deal with each other as though they were all of the same faith, and only apply the word infidel to people who go bankrupt. Here the Presbyterian trusts the Anabaptist and the Anglican accepts a promise from the Quaker. On leaving these peaceful and free assemblies some go to the Synagogue and others for a drink, this one goes to be baptized in a great bath in the name of Father, Son, and Holy Ghost, ... and everybody is happy.
Voltaire, Letters on the English, VI.
I very much doubt whether vegans and body-builders do metaphorically "hold their noses as they put their cash down" - when they go shopping, they are just customers.
I used to go to Whole Foods occasionally for supplements or esoteric ingredients unavailable elsewhere. I wouldn't say that I "held my nose as I put my cash down," but I definitely had a sensation of "the people at Whole Foods are not my people." So there is something to sixes_and_sevens' example. Now I think that either Whole Foods has more mainstream appeal or I've moved toward respecting Whole Foods' position on food, so I don't feel that way as much, but I still shop there only very rarely.
In the recent discussion on supplements, I commented on how weird an alliance health stores are. They cater for clientèle with widely divergent beliefs about how their merchandise works, such as New Agers vs. biohackers. In some cases, they cater for groups with object-level disputes about their merchandise. I imagine vegans are stoked to have somewhere to buy dairy-free facsimiles of everyday foods, but they're entering into an implicit bargain with that body-builder who's walking out of the door with two kilos of whey protein.
In the case of health stores, their clientèle have a common interest which the store is satisfying: either putting esoteric substances into their bodies, or keeping commonplace substances out of their bodies. This need is enough for people to hold their noses as they put their cash down.
(I don't actually know how [my flimsy straw-man model of], say, homoeopathy advocates feel about health stores. For me, it feels like wandering into enemy territory.)
I've been thinking lately about "allies" in the social justice sense of the word: marginalised groups who have unaligned object-level interests but aligned meta-interests. Lesbians, gay men, bisexuals, transfolk and [miscellaneous gender-people] may have very different object-level interests, but a very strong common meta-interest relating to the social and legal status of sexual identities. They may also be marginalised along different axes, allowing for some sort of trade I don't have a good piece of terminology for. The LGBT([A-Z]).* community is an alliance. Not being part of this community, I'm hesitant to speculate on how much of a weird alliance it is, but it looks at least a little bit weird.
This has led me to think about Less Wrong as a community, in particular the following two questions:
To what extent is Less Wrong a weird alliance?
On paper, we're all here to help refine the art of human rationality, but in practice, we have a bunch of different object-level interests and common meta-interests in terms of getting things done well (i.e. "winning"). I explicitly dislike PUA, but I'll have a civil and productive discussion about anki decks with someone who has PUA-stuff as an object-level interest.
Is there scope for weird, differently-marginalised trade?
Less Wrong celebrates deviant behaviour, ostensibly as a search process for useful life-enhancing interventions, but also because we just seem to like weird stuff and have complicated relationships with social norms. Lots of other groups like weird stuff and have complicated relationships with social norms as well. Is this a common meta-interest we can somehow promote with them?