Well, there is a way to display font awesome in RTE… it’s a trick. This is what I do for example by adding span tag and nbsp inside the i tag… but it will not preview in actual RTE view but it will when you hit preview or publish.
We posted a comment on Font Awesome in the last releases Release Notes. You actually don’t want to use the _tag (which means italics or emphasis not icon) for Accessibility reasons. A more accessible approach would be to use a span.
The system’s current HTML validator does require some type of content to be in the span.
As Nate mentioned, we have this issue backlogged as CMS-2404. We currently have this under consideration, but would like to convert this to an idea so that we may monitor its feedback on the community. If you find that this is a high-frequency use case for your team, please let us know.