Linguistics is just inseparable from conlanging. Once you start tinkering with language, you are going to wonder about how language works and get at least some interest in linguistics. Conversely, learning about linguistics is necessary if you want to try your hand at "exotic" language features (i.e. language features not present in any natlang you have a passing command of), or want to get better at understanding and describing your own work.
So discussions about linguistic theory shouldn't be banned per se. Still, especially since there's this Linguistics Stack Exchange site being built, I don't think all discussions about linguistic theory should be considered on-topic. Discussions about the merits and failings of Universal Grammar, or about the true nature of morphemes, do not bring anything on the table for us as conlangers, and may be even inflammatory. However, discussions about technical and/or applied linguistic topics (like: "what's the subjunctive mood?" or "what are active-stative languages" or even "what are phrase structure rules?") should definitely be on-topic, as they give us information that we can directly use in our craft (and usually will be asked by someone who wants to use that information for their own work anyway).
Also, don't forget that there still is an undercurrent of hostility towards conlanging in the linguistic community. Who says our questions will be welcome in the Linguistics Stack Exchange site?