Well, regardless of the notification type, it is impossible to know how users use the forums. As regulars here on Meta, and (in many cases) moderators or admins on our own sites, we know Discourse forwards and backwards, and will look at notifications (especially green and red) when we see them. We don’t know what the user on the other end will do.
I actually just had a discussion on a similar vein with a user on my site about who the users are. I was emphasizing that we really don’t know (in the case of an open site). We have users who do not speak the primary site language. We get young children who don’t quite know how to fully explain themselves, or read the guidelines. As @ccdw said, I generally don’t want to suspend or block a user. That is the last thing I want to occur. I want to help educate and steer users in the direction we want on the forums. I understand the concern about user privacy. I do. But we already have a huge amount of information about users, particularly admins, and I have not really seen any concern about this. If an admin wanted to, they can get the user’s last IP address, when they were last seen, their e-mail address, and a whole lot more. I fail to see how giving mods and admins the ability to see if a PM was seen would be any bigger of a privacy invasion then all the other data the site already collects. (And I don’t agree that there is a privacy invasion with the current information anyway).