G'day Snowspinner,
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David is right. We would be better off with disks overflowing with band vanity, original research, and poo jokes. Those don't do damage to the community and to the long term success of the project. The fact that we have to share the volunteers on Star Trek articles with Memory Alpha, and that the work can never be merged does. So does every other fork.
Although I'm a Trekkie myself, I think Memory Alpha is a Good Thing. It's somewhere for /Star Trek/ fans to list all sorts of non-notable trivia that doesn't belong in Wikipedia.
So what if we have to share /Star Trek/ volunteers? Actually, as I write that, I can see a counter-argument brewing: Trekkies might come to Wikipedia to put in their two cents on some meaningless piece of cruft (cruft, I say!), and stay on to write about other stuff. If they go to Memory Alpha alone, we've lost them altogether. But I suspect that's not your actual argument.
Why is it a Bad Thing to lose /Trek/-only contributions? Not just for /Trek/, but for anything where fans get a little too enthusiastic to judge encyclopaedic worth. Do we need a separate article for every planet mentioned in passing in the /Star Wars/ movies (or in the novels, or cartoons, or comics, or --- it'll happen --- fanfic)? How about minor /Digimon/ characters? I've noticed that, despite the existence of an *entire project*, /Harry Potter/ is missing vital information on what happened in a paragraph of page 421 of the latest epic, /Harry Potter and the Thingy-Riddled Thing/.
Would it be such a great loss to lose all of this? We're not talking Elf-Only Inn, here.