(See also this old discussion. I thought a new one would be valuable consider that the site has changed a lot since then, we have different tags and different on-topic expectations.)
The biggest danger with "biblical basis" questions is not that they are not answerable for being too broad, but rather that they are too basic. Really, a decent concordance, commentary, or bible dictionary can actually answer the questions. That said, I kind of enjoy them.
Let me start wit the con:
Ultimately, a Bible Dictionary can answer "What is the biblical basis for ... abortion, capital punishment, masturbation, nose-picking, etc." It doesn't matter if you are Catholic, Protestant, Mormon, or talking about any Christian denomination, the source texts that inform particular position are the same. The interpretation, contextualization, and relative importance of each may differ, but they are the same.
That said, you could say that ultimately there is nothing in Stack Overflow that isn't available in the documentation or the source code. The utility of C.SE for me - both in asking the question and giving answers - is that you can crowd source different traditions to find the source texts, so that you can go back and do your own theology.
Biblical-Basis serves as a signposts to the issues that should be considered - and the crowdsourced nature of SE is exactly what makes it useful.
We are, in essence, writing a Bible Dictionary here. At our best, we are gathering the issues and pointing back to the sources. And, unlike most bible dictionaries that favor one tradition, we look to several. I think its kind of fun, and that's why I like them.
@DavidStratton is right: Phrasing is really, really important when it comes to these. And, frankly, we get a lot of bad ones. But the good ones are the meat and potatoes of good theology, and if we want to be as indispensable to theologians as Stack Overflow is to programmers, this is exactly what we need to be.
Yes, it would be appropriate to tag those questions as
biblical-basis because that is ultimately what they are.
In these circumstances, however, the question might be too broad. The biblical basis for both arguments (in this case, for and against capital punishment) would sometimes be too much content for one answer. Hence, those questions should be split into the two "What is the basis for?" and "what is the basis against?"
When on simpler topics with less scripture to reference, I think they can stand on their own, however, the community should perhaps consider editing the question so that it clearly asks for the support for both sides. Answers that fail to provide support for both sides are incomplete and should not be upvoted and maybe even downvoted if they are very biased.