What is an opinion-based question and why should it be closed?
If this site were about physics or mathematics, I could understand why such questions should be closed. However, this site is about the English language which is far from being an exact science.
I suspect questions that are closed for being POB is often an excuse for reasons, which the close-voters don't want to reveal.
For example, I think the following questions are opinion-based, but they were highly upvoted.