We run WordPress at our K-8 school. We are very careful about moderating content. Our district policy is that nothing will appear online until it is approved by a teacher.
We have discovered that all users can 'see' comments in the moderation queue. They can't change or approve them, but they can see them. If someone were stupid enough to write something bad (logged in with their user account), they'd be caught by the teacher, but more damage could be done if their words were seen. Also, if someone shares their password or uses something easily guessable, that creates a whole 'nother problem.
I don't recall Contributors being able to see comments from other Contributors in the moderation queue. Is this how WP has always worked? We're on 2.1.2.