<p>Setting aside the specific thread on the main site that motivated this question, I think we can address the title question more generally.  </p>

<p>Namely, I think we should be mostly indifferent to what is posted elsewhere on the internet.  In this case, I think it shouldn't matter if a question is answered on Wikipedia, but we could apply the same principle to online statistics textbooks (e.g., <a href="http://www.itl.nist.gov/div898/handbook/index.htm" rel="nofollow"><em>The Engineering Statistics Handbook</em></a>), tutorial sites, (e.g., <a href="http://www.ats.ucla.edu/stat/" rel="nofollow">UCLA's statistics help site</a>), etc.  </p>

<ul>
<li>A good deal of the information on CV is likely to be duplicated <em>somewhere</em> on the internet.  Saying that we should only provide unique information would be too constraining.  </li>
<li>The internet is an unstable place.  What happens if the information elsewhere changes?  This is particularly an issue with Wikipedia, which may not say tomorrow what it says today.  (It is possible to permalink a particular historical version of a page, but this isn't commonly done.)  </li>
</ul>

<p>A question should not be closed because it seeks information that exists elsewhere.  (It should be closed if it seeks information that already exists on CV.)  If you have an answer that amounts to the same thing that is written elsewhere, I think you should be free to post it.  (That is, if it is your answer.  If you copy and paste, you should cite at a minimum, and under most circumstances you probably shouldn't post an answer that is <em>only</em> copied without any additional contribution.)  </p>
