That might be a bit stiff, for a number of reasons. If you meant that the judgment of a native English speaker is likely to be more in-tune with the language as the community uses it, yes, that's right.
But there are a couple of rebuttals that come to my mind as I read your statement:
1) You say "before" even trying to understand the subtleties, it's important to be able to speak it "fluently first." Actually, the two -- understanding the subtleties, and speaking fluently -- can only ever go hand-in-hand, after a very gradual step-wise learning process, a long phase of "becoming" that must be journeyed by everyone: you, me, and Bhai (we've already completed it) ESL students (who are on their way).
I take your point, but I would say from my experience and from that of my Spanish friends who speak a high level of English, that really getting to grips with the subtleties happens at a late stage of learning a language. I'm learning Spanish and I know that I am nowhere near the stage where I would be able (or wish) to use colloquialisms, slang etc. These are the sorts of things I mean by "subtleties". Once I believe that I can speak good, technical, grammatical Spanish at a decent level, then I will start to try to evolve a more "native" way of speaking.
2) This site mostly facilitates text-only communication. A few emoticons and links to short mp3 files can be seen, but you probably haven't heard many of the site's users actually speak. So you can't determine their level of fluency in speech. Many non-English natives have provided lots of great input on the subtleties of the English language. Indeed, some are so qualified to do so that their less common positions represent valuable new angles rather than odd points of view.
No, I have not heard any of the learners on this site physically speaking. But again, from personal experience, I know that I am much more able to express myself in a foreign language in writing than when speaking. My written Spanish, for example, is far better than my spoken Spanish because I have the time to consider what I'm writing and change it if necessary. If I were to post on a similar website to this in Spanish, it would give a false impression of my level - it would look far better than it is. I would expect the same to be true of many of the posters on this site.
I entirely agree that some of the posters on this site have brought some very interesting and entirely valid points to threads.
3) As a Ph.D. student in French in Paris, I often found -- often -- that I was able to explain the subtleties of French far more readily and correctly than native speakers, even educated ones. The reason? My focus at the time was that language, its difficulties, its subtleties. Most of my French classmates, who were math or Greek or Latin scholars, used French only in a colloquial, ordinary sense; they hadn't studied the literature, language or linguistics of French in years, and then only at a teenager's level.
All I can say about this is that I believe there is a huge difference between "learning a language" and "studying language". I think it's fairly easy to tell from the first post in each thread whether the student is still "learning English" or is actually "studying English", which I believe would involve much more complicated questions and, consequently, answers.
So, I find your comment overall is meant to claim superior authority -- an argumentation fallacy.
I actually have no idea what you mean by an argumentation fallacy, I readily admit. If you mean by this sentence that I have come across as superior and that is an argumentative standpoint, then I can only apologise if that is the impression I have given.
I seek to help where I can, and am more than happy to accept that plenty of people on this site have a far more in-depth knowledge of the language and its intricacies than I.