Well - to state/restate the obvious, languages evolve. English is changing every day, both in slow and gradual ways and in more immediate ones; some of these changes are welcomed, some are scorned, which is which depends mostly on the racial and socioeconomic status of the population where the change is seen to have originated*, but most eventually do make their way into standard lexicons, only to be pushed out later...
But how does this relate to a fictional language, like Tolkien's Quenya and Sindarin? As far as we know, the populations who "originally" used them aren't around, still using them, allowing them to evolve as they get used conversationally. In that sense, they are essentially dead languages; like Latin, or Old English, the point of learning them tends to be for very specific purposes of understanding either a document - Elvish poetry, for example - or a very, very specific subset of people. So to that extent, it makes sense to treat them... well, like Basque. You read the materials, you learn the languages, and then you use the language in whatever way you had intended - say, to write poetry or communicate with excellent geek friends.
The thing about the evolving process of Tolkien's work being published, I guess, in the way it seems they're doing it, is that it feels really artificial. Languages evolve in a very organic, messy, chaotic process, taking their cue from pretty much every aspect of the cultures they inhabit and other overlapping languages and cultures. If these languages are going to evolve they'd do so by being spoken. Which means, quite possibly and if enough people start picking them up, that they'll wind up evolving in two different directions - a vernacular, informal version (which, given the predilections of linguists and the folks likely to learn Elvish, is still going to be probably quite formal), and the version that the scholars eventually come out with in a few decades when all of Tolkien's work has been pored over enough times.
Either way, interesting times!
*of course, eventually the "bad" dialects of English, because they're spoken by a demographic generally disapproved of by gatekeepers, becomes cool and rebellious, and catches on, leading to essentially appropriation of a dialect by restless and rebellious youth, leading to backlash by the original speakers of said dialects, leading to a lot of confusion about whether it's okay to speak a dialect of English that's been used as a tool of oppression for social credit if you're never likely to be oppressed by those selfsame gatekeepers, or whether it's just a part of linguistic culture that dialects catch on over time. For more information, see ~23% of Tumblr.