Here's a cool feature of Apple's Mail I've never noticed before: "Reply" and "Reply All" toggle. This means that there's not such a strong reason to remove the "Reply" button from the toolbar as there is with, say, Outlook. If you chose wrong, it doesn't matter. You can keep changing your mind right up until you hit send, with no manual typing or removal of addresses.
That explains the otherwise mystifying presence of a "Reply" button on the toolbar for the window you're writing a reply in!