HTML vs Plain Text Email
Almost every email sent these days is an HTML email - even emails sent from Outlook or Gmail are HTML. This is because plain text does not support any images, inline links or the changing of fonts/text size and long links. It is well known that unstyled text does not convert.
Almost every email you see that looks like plain text is actually an HTML email that is styled to look like plain text.
Another reason not to send plain text emails is since they don't support images, you can't track opens with them, so you lose valuable reporting data.
Because of the above reasons we don't have any plans to support plain text alone.