I don't see any reason why we need a blanket policy for/against in-character answers. As far as bad answers go, we have two ways to deal with answers that are not up to snuff as far as the community:
Flag for moderation - this is for answers that are clearly not attempting to be useful, helpful answers. An attempt at a real answer doesn't seem like the kind of thing we should be punishing users for, just because of their chosen writing style.
Up/Downvotes - In general, I suspect most in-character answers are going to fall into the category of "on-topic, accurate, but not all that good" answers, which is what the voting system is there to handle. OTOH, if someone manages to write an in-universe answer that other users like, and conveys the information in a reasonable manner, I see no reason it shouldn't be allowed to stands on its own.
Using this specific answer as an example, it's a well-written, informative, and accurate answer, but my immediate reaction was "holy crap, way too many needless words, no way I'm reading this whole answer." I suspect any in-character answer is going to have a similar problem: the writer is going to spend a lot of the "real estate" in their answer just putting it into character, compared to the amount of useful information they're giving out. (I suspect there will be people who upvote it just because it's cute, which is their right, but the other answer is currently "winning" anyway, likely because it gives the same basic information in a much easier to digest form.)
Either way, if users find these kinds of answers useful and entertaining, they'll get upvoted. If users find them tedious, hard to read, or otherwise not useful, they'll get downvoted. I see no reason not to let that system keep working.
My only real objective concern with such answers is the difficulty in including references to back up your answer. It's very tricky to make references to specific novels, alternative information sources (blog posts, Q&A sessions, etc), or even many of the things that actually happened in the novels in the guise of a single character. How would you reliably work information into your answer that was only know to a few people, or that was presented as the internal monologue of a specific character, or that the author described happening in such a way that there were no witnesses? It's certainly possible (@neilfein dropped a reference to the Wikipedia entry on Ringworld into his answer) but too many of them will start to defeat the whole purpose of an in-universe answer.