Perhaps. But I imagine there are many people who wouldn't like the idea of the activity of their private messages being so monitored and reported. While it would be good to know if there was a technical glitch and the message never made it to the recipient, how would knowing whether or not it was read be of any use? If it hasn't been read, there is no other way to get in touch with them and let them know there are messages waiting for them - so sending more messages won't help. And if they did read it but chose not to respond, well, that is their right. You can't force them to respond and sending more messages, when you know they don't want to respond, just seems like borderline harassment.