This is important for both moderation and communication for editors that it should be implemented into MediaWiki.
- Not everyone has or is willing to give out an email address. This can make private communications difficult. For example, if I had a question for an editor or just wanted "general chat" on a wiki, I would have to use email, which has the problem that it can potentially expose your email address. And if I or the other editor don't have an email address, then I cannot communicate via email. Private messaging solves this problem.
- If an administrator wanted to coach someone without causing public embarrassment, private messaging is good for that. For example, it would be embarrassing to send a warning template to an editor for something as trivial as accidentally making a personal attack. Sending a private message avoids that embarrassment.
- Emails do not support wiki markup and are sent in plaintext. Private messages could, in theory, support templates and linking and other forms of MediaWiki markup. This can add rich-text to private messaging.
Of course, users that abuse the private message system should be able to be blocked from sending (but not receiving) private messages.