Frankie, not just useful for English, but for any online course. It provides a way to give proper feedback with a little more speed than the returning of documents, with more elegance than the annotation tools in either word or PDF - or replies to posts in forums. I've used this on the SCoPE forums.
I decided it was worth writing up so I created a Marginalia section in the docs.
I'm a traditionalist here. If I need to work online, I like two panes: highlighting in a left text area, and comments linked in colour code in the right. I dislike postits, silly small text sizes, tons of social media intrusions. For simple feedback to a student/co-worker (and I'm not meaning huge networked learning contexts) this scenario suits me.
I guess what I want to focus on is the text in front of me. Marginalia (even if it does not have the bells and whistles as you say) helps a lot.
-Derek