I recently came across a question in the review queue that had attracted a couple close votes as "primarily opinion based" (What is the best practice for aligning table content for text/number mix).
Sure, I can see how it could be answered with "it's just a matter of opinion" and just close it as such, but the answer that was provided was more useful than a silent close-vote. In this case, I believe it's beneficial to the OP to note that the reason for aligning table contents to the right is so the decimal point lines up and it's easier to compare numbers of different lengths, and thus, in this case it doesn't really matter either way.
To contrive another example, imagine a well-written question that asks "Should I use pattern X or pattern Y for this type of application?" Unless there's a clear-cut better design for this use case, this seems like a question that many users might reflexively flag as "primarily opinion based". In reality, however, the poster might greatly benefit from an analysis of the pros and cons of pattern X and pattern Y, even if the answers conclude that it's just a matter of opinion.
In some cases, even though the answer might ultimately be just a matter of opinion, is it fair to leave a question open when there's more to be said?