title: Is there any reason to not set 'git fetch' to always use the --prune option?
body: <p>Using git fetch --prune deletes local remote tracking branches when the branch on the remote machine has been deleted.  Setting remote.origin.prune to true using the following...</p>
  
  <pre><code>git config --global fetch.prune true
  </code></pre>
  
  <p>...makes using the fetch command always implicitly use the --prune option. </p>
  
  <p>I am putting together a best-practices/introduction to git for some developers in my group who aren't quite familiar with it. I want to be sure I know this is not a dangerous behavior before advising them to do so. I at least give them a heads up of what to watch out for if there is some extraneous mishap case. </p>
  
  <p>It doesn't seem like this is a destructive operation because it doesn't delete any local (non-remote) branches. It also seems like this is a great way to not build up remotes that aren't in use anymore without periodically specifying git fetch --prune or git remote prune. </p>
  
  <p>If this is all true, why is this not the default behavior for git?</p>