If a legal question is at all specific, I agree that it would be best answered by lawyers. (What's more, lawyers practising in that particular jurisdiction. Many questions and answers seem to assume US law applies everywhere in the world, when in fact the laws in individual US states, let alone other nations, often differ considerably.) But there are many situations where in practice a questioner is unlikely to find a lawyer able and willing to answer their question in a timescale or at a price that is practical, and even more situations where all they need or want is a simple answer.
I really do take your point about the danger of creating a blather of unqualified "legal opinion and plain guesswork." Answers on legal topics from non-legally qualified people should be given with care and with I-am-not-a-lawyer caveats if it looks at all likely that the questioner might be heading into trouble.
Nonetheless, here are some points in favour of answers to legal questions from Writers SE members who are not lawyers:
1) The questioner is asking here and now. A reasonably good answer here and now is probably much more useful than a perfect answer later from someone else somewhere else. Asking here is the equivalent of "asking around" your friends, colleagues and family to get a handle on a problem, a traditional and reasonable first step to solving it - but better.
2) The specialized legal forums you mention can be helpful, but have several downsides.
They don't exist for all jurisdictions.
What is a minimal fee for some people can be a real problem for others.
I've never quite been sure how much to trust the free ones. I don't mean to be cynical, having often been amazed by the helpfulness of obviously professional people on the internet to me, a stranger - but the fact remains that just because someone's posting on a legal forum doesn't make them a lawyer.
They are often much slower to give a response than Writers SE. There are fewer people available to answer questions there than here. General-practice lawyers are reluctant to answer questions about libel law, for instance.
3) For developed countries the broad principles of law relating to writers are not so obscure that only lawyers can advise. Askers can be safely pointed to good general descriptions available from respected sources on the internet regarding copyright law, contract law, defamation law, and so on for most developed countries. For those needing more detail, there are legal books that can be safely recommended. My copy of Robertson and Nicol's Media Law cost me £35 when I got it, which was painful but still only a fifth to a tenth of the cost of one hour of a barrister's time.
4) Which leads me to the next point: it just doesn't seem right to perhaps leave someone in fear when I know perfectly well that they are worrying unnecessarily, or to give someone the impression that they must get expensive professional advice when they really don't need it. For most legal questions I have seen on Writers SE a general answer is sufficient. The questioner can be pointed to more qualified sources if need be.
5) Reading legal questions and their answers educates the readers of Writers SE on the broad outlines of those areas of the law which every aspiring writer should know something about. Furthermore the questions and answers are often interesting. The law provides some great stories, which ought to be welcome on a forum for writers.
Just to reiterate: I do take the problem of the blind leading the blind seriously. I have been threatened with a libel suit myself, and this was in Britain, the libel lawyer's happy hunting ground. Any time it starts looking hairy I'll be the first to say that the enquirer should be asking experts not amateurs.