Does it matter? The default is HTML, the majority of mail is HTML (or atleast multipart). So why does it matter if GMail defaults to what the majority is doing? Everyone (99.9999% of people) can receive and send HTML, so just do that, it's the informal standard for mail now.
It's great that you think that it matters but I don't think that what you think that it matters matters in practise, really, the world is HTML email now.