I don't really think we need negative points. If you see a user with no postive points you can just take that to mean that they don't really contribute much positive feedback. Negative points allow poeple to abuse the system more. Another possibility, as someone else already suggested, is to show a user's positive and negative points. This would be more informative than just showing one value.
But I do agree that poeple should not be allowed to give negative points without giving a reason.
