1. People were using them like private messages, which is not the purpose of them, takes up server space as there is no way to limit them, and also led to:
2. People were saying things about other members in them, which those members were then seeing and thus:
3. There is no possible way for the moderators to read ALL the visitor messages in the same way they read all the posts on the board, and it would be an extreme waste of their valuable time.
4. This isn't Facebook, it's a place to discuss Buffy and Angel.