It has been a while since I have done my "mail.app" tests, I enjoyed the process, thank you. You are absolutely correct, with the current version, at least, [Version 9.3 (3124)] the message does not include any "rich text/HTML" until a font is actually changed (font, size, color), an image is inserted or some other element is inserted like a table. I don't know if this is a new behavior or my memory is faulty. Regardless, when there is no need to add the "Rich Text/HTML" it does not. But change Even one character, all kinds of HTML code appears when viewing the Raw Source(View-Message-Raw Source). Even though the Message Format is set to "Rich Text"(Mail-Preferences-Composing), unless some formatting is done, Plain Text is sent. Even though I don't use mail.app, I think it is very well written. The amount of bandwidth saved by simply not including unneeded encoding is got to be massive. I am willing to bet no other mail client is this smart.
I did also verify when using a Signature, HTML is always sent unless "Always Match my Default message font" is checked (MAIL-Preferences-Signatures). When "Always Match my Default message font" is checked, plain text is sent. It really should say "always send plain text". So a work around is to always include a signature and type within that signature and your intended font will be sent. Just make sure "Always Match my Default message font" is not checked and your signature is set to the font you wish to be your message font sent.