Comments are definitely "second class" posts.  They're intended to be used to ask for or to provide clarifications to the main post that they're on.  If they don't improve the post that they're on they might be deleted immediately, but you should always consider your comments expendable.  [Obsolete comment threads will be deleted][1] (eventually).  Critical information should not be left in comments, but edited into a question or answer.

Remember that it's not just moderators that can delete comments.  Enough [flags from regular community members will *automatically* delete comments][2] without moderator intervention.  


  [1]: http://meta.stackoverflow.com/questions/77373/should-moderators-delete-obsolete-and-resolved-comment-threads
  [2]: http://meta.stackoverflow.com/questions/15290/are-all-the-1-comments-really-necessary/15292#15292