- How to decide whether an answer would be useful to learners or not?
- Is it our duty to decide whether an answer would be useful to a learner or not, considering that there are different kinds of learners and learning needs are not the same per person?
Yes, I think you should judge whether or not an answer will be useful to a learner. You're right that there are different learners with different needs. But keep in mind we're not just answering OP; we're helping future visitors as well. In answering the questions, I think we generally have an idea of the kind of learners we reach. Or rather, we try to give answers that will improve the typical learner's fluency, or help them attain a "native" level of speaking.
In the linked post, OP wants to know
What's the word that means spitting while talking?
and a user gives
It's sialoquacious or sialoquent.
Users rightly noted that the word is rare. The typical native speaker would balk at the usage and possibly wonder if it was simply a non-native speaker's mistake. This not the kind of scenario we want to set up for the usual learner.
OPs should be provided with special usage notes if needed. For example, if the word is
- really formal or informal
- rare, obscure
- regional (AmE, BrE, etc)
- vulgar, mildly vulgar
OP should be informed of this. I think we're generally good about providing these kinds of notes.
Referencing the question and answer above, yes, you, I, and others might appreciate the word, and indeed the word is accurate, but in helping learners sound like native speakers, is this a good choice? I would say no. I think that's a good way to judge whether or not an answer will be useful: Is this accurate and does it help learners sound like native speakers?
Often, askers will take answers on faith and assume the usage would be clear. But we should strive to inform learners whenever that is not the case (as in the linked example).
I think our main concern (aside from the frequency of the word use, which should be indicated) is knowing whether the word is valid or not? Or whether the references are valid or not?
I'm not sure the validity of a word is the main concern. The validity and the currency are equally important, I think. In other words, being valid and helping learners sound fluent are equally important.
Also, the references being valid is certainly important, but I'm not sure about that being the main concern.
Or maybe we should remove word requests and phrase requests here and have them all in EL&U?
No, I think there is definitely a place for the requests here. As I suggested, the kinds of answers we provide are tailored for learners, to help them improve their fluency. Occasionally, I see word requests from ELU and the words they bust out are obscure. It would be hard for learners to navigate through that, as they typically wouldn't have the knowledge to assess all the quirks and caveats I mentioned above.