When pressing a Reply link under a message, the UI shows an empty text box to type the reply, with an option to press Quote button above to add the quoted text of the original message. I think, the quoted text should be added by default, as you click on the Reply link.
The message will be eventually sent to a mailing list. Without quoting by default, the current UI encourages the user to not quote anything at all, and such messages are difficult to read in an email client as they are lacking any context. We have a section regarding proper quoting in our discussion guidelines, and it would be nice if the UI helped the user to follow them.