I agree 100% with Box on this. Typically you scan a thread rather than reading all posts, and very often there are multiple discussions going on side by side in the same thread. Leaving the quote inside the reply, as the replier intended for it to be, makes it much, much easier to follow what is being said.
Also the lack of transparency is just annoying. You do something and get a seemingly arbitrary result. If you really think about it, and make some research or experiments of your own, you can figure out exactly what happens when, but as a user you shouldn’t have to! Software should follow the principle of least astonishment, and users should be able to focus on the task they want to get done, not the software itself.