Exchange generates Tracking logs in these format for different component of the server.
MSGTRKyyyymmdd-n.log (Hub / Edge )
MessageStats only looks at the Tracking logs from the Hub/Edge servers.
Mailbox Tracking Logs are not useful logs for MessageStats and they are ignored in the processing of Exchange Tracking Logs. These are the files that has the name MSGTRKM at the start of the file name.
There is no need to incorporate the importing of these logs as the same information is found in the Hub servers tracking log.
The tracking log files on the Mailbox Role server only have submission events (i.e. when a user creates a message using Outlook and clicks send) and the entries in the mailbox role tracking log are missing key envelope information, such as the Originator, Recipient and Subject line.
With Exchange 2007, any submitted message must go through the Hub Transport server so that it can be routed to its destination. For all submitted messages, the hub transport role server will see the message. The tracking log for these servers have all of the information that was missing from the Mailbox Role server.
This is why MessageStats does not currently gather the MSGTRKM files.
We may in the future if Microsoft pushes more information (or events) into them that would be of benefit in MessageStats Reports. But for now, the Hub Transport Role tracking logs (as well as the Edge Transport Role servers) have all of the information.