If someone hasn't read your message in a month that means a different thing than no response for a month. Some would assume you said the wrong thing, etc. Also remember Facebook is software- which can fail. Maybe this is also a way to help reduce message delivery failure- with email you might get a message bounce back if the send failed. If a Facebook message failed to deliver due to bugs, hardware failure, etc how would a user know? If they don't know they probably won't complain to the FB message API developer/manager who has a "can't reproduce", "intermittent", "sometimes occurs" bug.
If it's important you'll find another way to contact them.
The logic behind using it to show if they fail or not doesn't work. The system that would identify if a message was sent would be tied into the messaging system; They should have this if it's possible, though they don't need to make the results known.
