RTF is an pretty established implementation standard (which is displayed and interpreted differently in almost every application that supports it, because there are lots of application- specific extensions to it).

There's no way to display xml/html/xhtml in a standard way either. Each browser shows it differently. ;-)

BTW I'm not a fan/supporter of RTF so to speak. I'm neutral with RTF. I haven't really had any bad experiences with RTF, nor really excellent ones.. except that it seems to load fast when I remember using it. Now, everyone seems to throw up a web page instead of sending RTF files. At least that is what I do.. or I send plain text.