This might be a silly question, but was there ever any thought put towards using ES6 Template Strings rather than the JSX syntax additions? For example:

    class OuterComponent extends React.Component {
      render() {
        return (
          <MixinComponent>
            <InnerComponent />
          </MixinComponent>
        );
      }
    }

Being replaced by something more like:

    class OuterComponent extends React.Component {
      render() {
        return JSX`
          <MixinComponent>
            <InnerComponent />
          </MixinComponent>
        `;
      }
    }