There are threads on other sites (the TVTropes one is the biggest, I think, but I know the xkcd forums have a thread, and I'm sure others do as well). Part of the value of having HP:MoR threads here is it makes it likely that people who come here for the MoR threads will stay for the rest of the site- but I agree that the karma on them is atypical for karma on the site, and decoupling it would have some value (but I suspect higher costs than value).