<blockquote>
  <p>What is the actual harm done by an extended discussion in the comments?</p>
</blockquote>

<ol>
<li><p>An extended discussion in the comments obscures any other comments that do not belong to this discussion. Simply because they become hard to spot. Only 5 comments are visible without "expanding"; if there are 25 comments of a discussion between users @A and @B, then very few people will ever see a 26th comment by @C that might contain important criticism of the answer, or an important insight. Worse, @A and @B might go on to post another 25 comments after @C's one, in which case it is practically guaranteed that nobody else will ever see it.</p></li>
<li><p>Extended discussions in the comments tend to escalate. Yes, I can imagine a civil and interesting "extended discussion" in the comments, but very often they get ugly. Even if nobody ends up raising their voice, at least one of the parties will often feel offended. SE wants to prevent this from happening by stating that it is not a platform for debates.</p></li>
</ol>

<p>If a discussion is ongoing, appears meaningful and remains civil, then I prefer moderators to let it go for some time before eventually moving the whole thing to chat when it's over anyway. However, if it starts to lose civility, I think it makes sense to move it away asap. That might have been the case for the discussion that has triggered this thread.</p>
