I have now read the blog at http://blog.stackoverflow.com/2011/08/the-future-of-community-wiki/
which seems to answer that yes, CW's only actual function is "don't award reputation here". (although it's not clear that this is how questions should be identified for CW)
I strongly dislike that but I obviously won't go against official policy.
As an expert mathematician, I spent my 10,000 hours on pedagogy and it saddens me that education or basic mathematical understanding questions are often marked as 'soft' or 'community wiki'. It seems to be a way of showing disrespect to the field of mathematical pedagogy.
This particular question is popular perhaps because it's accessible: everyone can have an opinion. However, I don't agree that popularity or accessibility means the question and answer is of less value to the community: there is still a significant level of expertise that is needed to accurately answer the question.
There is some issue that questions which are inaccessible and require expertise are relatively undervalued (because people very rarely vote on those) - however, I don't think this is resolved by making accessible questions immediately worth 0.
The 'sister' question used as an example is not a joke, or a fun question, and I believe people are upvoting it because it has value here which it deserves to retain.