I'm relatively certain (>95%) that Dymitry/Private_messaging gets special treatment (ie. deletion) because the admins consider him a troll. The point of deleting even his reasonable comments would be to get him to stop commenting at all. I'm not aware of any other LessWrong users who are considered trolls by the site admins.
If you don't like participating in threads where things randomly vanish, stop replying to trolls.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.