Normally each node has a status field "published" which is kept with the same node table.
It would be nice if messages also have that kind of field. We would use it to indicate if the message is read or not for example.
This can also be achieved by adding a field by "manage fields" interface of a message type, but since they are kept in a different table, it would be slower. Also if I have 8 message types for example, I would add this field for each one.