A few months ago we had a discussion about the use of the PLM Monitor Mode.  According to the PLM guide, 

"In Monitor Mode, the IM will also notify the host of received INSTEON messages that contain a From Address matching any INSTEON ID in the IM’s ALL-Link Database, even if the To Address does not match the IM’s INSTEON ID or the IM does not belong to an ALL-Link Group associated with the message."

In practice, none of us was able to achieve the results expected.  The PLM would acknowledge being place in monitor mode, but no difference could be detected.

Over the weekend, I was working with an EZFlora device which is able to send "broadcast messages" on valve changes.  Initially I was unable to see the broadcast messages.  After poking around the interwebs, I discovered a few posts which noted that the PLM will only pass broadcast messages through if it is in Monitor Mode.

So I enabled it, and amazingly it worked.  I started receiving messages such as the following:
28/03/2014 14:34:21  [Insteon_PLM] DEBUG3: Processing PLM raw data: 025028aab34000008627a3
28/03/2014 14:34:21  [Insteon_PLM] DEBUG4:
         PLM Command: (0250) insteon_received
            From Address: 28:aa:b3
              To Address: 40:00:00
           Message Flags: 86
                Message Type: (100) Broadcast Message
              Message Length: (0) Standard Length
                   Hops Left: 1
                    Max Hops: 2
         Insteon Message: 27a3
                            : Insteon message type not decoded
It appears that this message structure does follow the Insteon Specification.  But the best I can tell, no other device on my network has ever sent a true broadcast message.  Note that these messages are similar but distinct from "All-Link Broadcast Messages."  The Insteon Thermostat claims to send broadcast messages, but in actuality, these messages are direct messages.  

So Monitor Mode does do something, just not what the documentation would suggest.