Many things strike me when I read more about Tolkien's relationship with the world of language. Quite frankly, the one that strikes me most often is "WOW THIS IS AWESOME THIS IS SO COOL", but that's hardly a blog post.
One thing that really interests me is how much, and how unusually among his peers, Tolkien considers old languages in a distinct way. It's a bit hard to describe, put perhaps one could say that he treats them as equals, rather than as quaint curiosities. He takes them on their own terms, as much like those who naturally spoke them did as he can, and thus gets familiar with them as a friend to a friend, rather than an anthropologist to an aborigine.
I think this must be what makes Tolkien so competent both as a philologist and as a philology-inspired writer. Rather than having a "furtive or at least apologetic attitude" towards his enjoyment of language, as he says many other language scholars do in his "English and Welsh" essay, he is, as Carpenter says, "excited" about language. If you had a certain good friend, would you carry around a notepad and dispassionately analyze his motivations, record his quaint customs and rites, sketch his traditional garb, etc.? Of course not! Yet I put it to you that you will know your friend much better than you would doing such things by genuine human interaction as equals*, and so too for Tolkien and language.
This attitude of Tolkien's is only further confirmed, and its success indicated, by his creation of the legendarium. Someone who studies so-called "dead languages" as something just that--dead--can hardly get as much insight into them as someone who interacts with them as living beings. Just as Tolkien links people through the ages, as with his Eadwine/Aelfwine pairs, he perceives the links between old and new languages in different ages. And perhaps it is less accurate to say that he links people and languages through the ages, and more accurate to say that he links the ages with people and languages. Most people would say that Eadwine and Aelfwine, Audoin and Alboin, Edwin and Alwin are nothing but completely separate, unrelated except genetically, and very dead pairs of men. For Tolkien, while they are different people, they are also somehow aspects of the same pair, and each sufficiently alive to be interacted with as with a friend, despite the fact that they may happen to have died in the meantime. So it is with language; something like the Middle English of the West Midlands may no longer be anyone's "mother tongue", but Tolkien treats it as quite as alive as any modern language.
Which brings us to the matter of mother tongue vs. native tongue. ("Mother tongue" is the one you learn first and "native tongue" is the one that you have a special connection to, right?) Again, the analogy of the friend versus the anthropologist applies perfectly. A scientist can study any research subject he has reason to; he may prefer one to another, but that is not very significant. A friend, on the other hand, cannot simply choose who he is to be friends with. He may attempt friendship with, or shun, any person he has contact with; but the fact remains that there are some people with whom his nature will allow and encourage him to be good friends with, while there are others--not necessarily bad people--whose natures are simply incompatible with being a good friend to this person. So it is with Tolkien and language; by giving up the possibility of relating to very many languages (not a large loss relative to his gain), he was able to relate on a personal and, one might say, intimate level with the "native" languages (Old/Middle English, Welsh, Finnish, etc.; but apparently not Gaelic) that were true friend material for him; that excited him.
Wait, did I say that "WOW THIS IS AWESOME THIS IS SO COOL" was hardly a fit topic for a blog post? Hm. Well, I retract that, as I seem to have written my whole blog post on the fact that Tolkien's "WOW THIS IS AWESOME THIS IS SO COOL" approach to linguistics gave him unique insights galore. As I'm the kind of person who gets ecstatic for the whole day when I deduce a subtle etymological link in some language I know, or reads the Wikipedia page on Varangian runestones trying (and partially succeeding) to read the Old Norse without actually knowing it, I find I'm liking this guy more and more. Where do I sign up for Philologist Academy**?
*or by tying him up and holding him over the volcano's edge, if you're Xiang Yu, but I hope you're not
**I'm only about 1/3 kidding