Yes. I agree that if a question is about the beliefs of a specific group, the logical basis for that belief would have to be the texts considered authoritative by that group.
If the purpose of this site were to evangelize, one could certainly object by saying "I won't believe this unless I see it in my authoritative text"...but the purpose of this site is not to evangelize.
The Sadducees tried this millennia ago -- they would ask for a belief to be justified using only the Torah. The other sects of Judaism, of course, believed lots of things that are not in the Torah, because they accepted the teachings of Isaiah, Jeremiah, Ezekiel, etc. This exercise tended to be unproductive.
A natural corollary, then, would be that if a Papal bull, an ecumenical creed, the writings of Augustine, a statement by Aquinas, Luther, Calvin, Joseph Smith, Charles Taze Russell, Emanuel Swedenborg, etc. effectively articulates a theological principal believed by a denomination, on an open site like this we will have to accept that both of these statements are true:
- Many Christians will reject that source but
- Those sources are certainly in scope for explaining the views of the denomination in question
Nobody is required to believe what is said in Tobit, 1 Clement, or Moroni, but we share this site with people who do. They are welcome to cite those sources in explaining their beliefs.
Post-script per the ongoing discussion
If I understand correctly the comments that have been made here, no one sees a need to eliminate the biblical-basis tag, though some see a need for clearer scoping.
The following may be an effective way to ask biblical-basis questions of faiths who accept more than a 66 book canon:
- What is the canon basis for this Catholic/Orthodox belief, and to what extent is it dependent on Deuterocanonical texts?
- What is the canon basis for this LDS belief, and to what extent is it dependent on Restoration texts?
This opens the door to highlighting passages in the canon that is shared by the asker & the answerer, without artificially restricting the answerer to an incomplete response.
A touch of humor to evaluate some alternatives
1 - If the answer to the OP's question is "no", we should probably just have 1 question on the site that goes like this:
Question: Do denominations which accept more than a 66 book canon believe things that are not explicit in the 66 book canon?
Answer: Yes
And then the matter is resolved once and for all and we can move on.
2 - Alternatively, I think it would be fun to ask everyone to argue all of their theological beliefs using only the Song of Solomon as a source. =) =) =)