Okay, it took me a while to figure that one out cause I was certain that I repeatedly had to fill in the username manually: the issue exists only when replying to the first post.
Yes, I think the current practice of putting a reference into the body is good and should not be changed. The reason why I would like to also have he title populated is simply that I can’t submit the message if the title field is empty. If I’m not happy with the default title, nothing will stop me from changing it but most of the time it would simply be a convenient time saver.