I can see a usage case especially in the native environment for Facebook -- college students. So, you tell someone, "Hey, remember, you're playing with us in the Frisbee competition tomorrow over in Rochester. Meet me at 9am, North Campus." The next morning before you drive off, you check for messages and they haven't responded, but it's marked as "seen", which is some small comfort -- you still don't know that they woke up in time, but otherwise you can expect them to remember.

It's actually also possible to implement this in email, although the loading of remote images is disabled by default in Gmail. Any email which contains a link can point to a web service which notifies someone, "hey, this page was accessed."