I could see "@User123 has entered the thread" working in the smaller forums I was a part of where I knew everyone. phpBB/vBulletin show a "Users reading this thread: ..." list for that reason, and there were popular plugins for things like "@Foo and @Bar are writing responses to this thread". It helped make the place feel more alive, especially when you were in an intense discussion. It was nice to know the fker was working on his next big quote-by-quote debunk of your post.

But getting a notification when unregistered guests click into a thread doesn't seem useful to anyone.