I strongly support this. The commentor or poster mentioned is likely to be both interested in the subject matter of the conversation and in how their ideas are being referred to. I think it may function almost like a citation, just adapted for an online social site. Personally, I'd appreciate receiving these to protect the integrity of my ideas and words. If it's being used in a way I didn't intend, having the notification would allow me to correct any misunderstandings.
Or, of course, we could all just have an informal agreement to tag people who's posts/comments we reference.