Some of the most memorable stories of Buddhism come from outsiders questioning monks or teachers (be it in hostility or out of a genuine spirit of curiosity), and these stories are often used as teaching tools. Buddhists, especially in Zen, have a strong tradition of dealing with such questions, even when they seem too broad or require an indirect answer that relies on analogy.
We would all be much the poorer if "Why did the ancestral founder come from the west?" had been closed as "too broad" before Zhaozhou could answer.
For this reason, I think we should use a lighter hand than usual when considering closing questions like those you point out. In both the examples you point out, I can imagine the existence of an answer that's both concise and satisfying, and I think such an answer would really help the site.
Even so, I'm not suggesting we should just allow anything. In particular, I think we have to match this attitude with a ruthlessness about duplicates. A hundred questions of the form "does X have Buddha-nature?" wouldn't help anyone. Just remember that closing a question prevents it from being answered, so a vote to close represents a belief that any answer to the question would harm the site.