I voted no, but hidden by default is a vaguely decent compromise. What would make it even better is if clicking "Quick Reply" next to a post would make it become a child of that post. (Actually, it may -- I haven't tried it -- but I doubt it.)

Edit: Okay, I tried it. It sets the QR post's parent as the latest post.

Edit edit: Okay, what would be really cool is a hidden QR for each post, so that replying doesn't require loading a new page and the heirarchy would be kept. Not that I think that that's feasible.