Rich text is a Microsoft invention, which is only used purely in Microsoft-based networks. Both Outlook and Exchange support rich text encoding, but many other e-mail clients do not. If rich text is not supported by the client the formatting will default back to plaintext and remove any formatting. Rich text offers some formatting features, but generally offers nothing more than what HTML encoding offers.
Since more people support HTML-encoded messages than rich text, this encoding style is rarely used. It makes me wonder why Microsoft even consider inventing something that no one uses. All of my emails are either in plaintext or HTML.