I haven't been around for too long and answered that オタク question myself, but at that point I hadn't seen any questions like that yet. I think that these questions do not add much value to the overall information on this site after a certain point.
Some questions have been too specific at some point to be explained with information provided in previous answers and there is still a possibility that some new question pops up that can't be explained by linking to old answers, but I believe that this is the solution. The question was originally about ダメ but the title has been reworded to be more general.
This solution doesn't address the fact that most people will google "why is カタカナ written in katakana?" and not "why are some Japanese words written in katakana?". Unless Google gets smarter and learns to interpret any word written in katakana as a question about katakana in general, we will also need to list the actual words somewhere on the question page so that googlers will find their way to the answer. Closing a question about a specific katakana word as a duplicate without adding the word anywhere else will lead to new questions about the same word.
That said, these simple and intriguing questions seem to attract many visitors. I don't know how the "Hotness" of Hot Network Questions exactly works, but I think we can all agree that a question about the etymology of a word most people on the network can't even read won't end up there. Judging by views, I believe that the オタク question was one of them and today's question seems to be "Is there a term for Japanese characters as a whole?". If you think of these questions as a way to attract more people to this site, they aren't too bad. It doesn't change the fact that information about general rules for katakana spelling will be scattered around the site, though.
So will the question become if we should improve the answers to the question linked earlier (or create a community wiki for it) and point to that when new questions come up without closing the question as a duplicate? I'm not sure how feasible it would be to maintain a list of katakana words and hope Google will point to that. There are many rubbish sites that list common words in hopes that the site will show up in Google searches. Google must put some effort in ranking these sites down in results.