1- Often applications couldn't care less about what's in the rich text. When they do, you can still get it for 'em.
2- Document preview in Notes can be abyssmally slow, when all you really need is a half-page image load independently. Advantage: Outlook.
3- Even omitting attachments can give performance a big boost.
4- Making it a developer-programmable option allows seamless support for all existing applications.