That's a fair response, but you're presupposing a client that behaves as it's supposed to. To my mind, the problem it solves is familiarity - average users are

That's a fair response, but you're presupposing a client that behaves as it's supposed to. To my mind, the problem it solves is familiarity - average users are accustomed to "file.html", not "file(oh hey, send the text/html version please)". All of this is heavily influenced by who your users actually are, of course.