I agree with you that languages and language varieties are both adaptable and multifunctional. I will add to the mix of comments on this discussion the observation that we are all adaptable and multifunctional in our individual language use--even the English (that is, those from the former Great Britain--oops, just kidding, folks) control more than one variety of the language. You know those radio shows, all news, all the time? We are all multidialectal, all of the time, mastering levels of formality, adjusting content to context in speech as well as writing. And our language is not limited: It can grow and mature, adapt, and in some cases even atrophy. We lose and gain words, expressions, pronunciations. Language is creative: We can manipulate it to match a new concept, or a new invention, or simply to tell a joke. And we seem able to use language to discover new concepts as well.
Language is also a kind of self-regulating system, adjusting system-wide to its users' needs and whims, and it is also a system in which the users attempt formal and informal kinds of regulation, establishing standards or trying to do so, making judgments about our own language use and that of other people (in the words of the radio commercial, "People judge you by the words you use ..."). I find that formal attempts at regulation--language laws and language policies, often fail to achieve their ends, or achieve ends somewhat different from what may have been intended. I find that we all establish linguistic standards and try (but often fail) to adhere to them; but interestingly, our standards don't align. Our linguistic use and preference vary slightly from person to person, so the system remains in motion.
I also find that when it comes to discussing language, we have a significant user-vs.-expert problem: Users of language claim a certain amount of expertise, and rightly so; but they also tend to reject or challenge what experts have to say. Again a complicating, subjective factor is that experts are also users, a situation that both compromises and informs their expertise.
Is it your experience that experts on language are challenged with greater regularity by the general public than, say, experts on math (let's leave evolutionists out of this for the moment)?
Language is a complex issue as well as a complex phenomenon. When language is perceived to be a public problem (bilingual education, poor writing test scores, nonstandard usage), the public seeks simple solutions: Make English official; teach more grammar; emphasize correctness. But simple solutions don't seem to work for complex problems, either in language or in medicine.
By the way, to return to the subject I introduced this morning: In addition to the grammar-gene study, a colleague reminded me of a medical study a few years ago that showed writing could improve the immune system by actually stimulating T-cells. And I myself remembered the so-called "Nun study," which claimed that sentence complexity in writing could be used as a predictor of Alzheimer's disease later in life (in essence, nuns who wrote simple sentences as novices were more likely to be Alzheimer's victims in old age; those writing more complex sentences were less likely to contract it). I've downloaded the JAMA article, and while I haven't had a chance to read it yet, I did notice that the experiment was structured so that some participants were asked to write about their most stressful experience, while the control group was asked to write about their plans for the day. Those writing about stress showed the most improvement.