Q. こんなのいつ使うの

A.
パフォーマンス無視できる
ビルド環境用意するのが面倒

実装例

index.html

<script src="//unpkg.com/requirejs/require.js" data-main="config.js"></script>

main.jsx

import React from 'react'
import ReactDOM from 'react-dom'
import {Main} from 'jsx!main.class'

ReactDOM.render(
  <Main cont="hoge" />,
  document.querySelector('#out'))

main.class.jsx

import React from 'react'
export const Main = props =>
  <div>{props.cont}</div>

config.js

require.config({
  paths: {
    babel: "https://unpkg.com/@babel/standalone/babel.min",
    react: "https://unpkg.com/react/umd/react.production.min",
    "react-dom": "https://unpkg.com/react-dom/umd/react-dom.production.min"
  }
});
require(['jsx!main'])

jsx.js

(() => {

const callMethod = name => obj =>
  obj[name]()
const transform = babel => code =>
  babel.transform(code, {presets: ['react', 'es2015']}).code
const format = code => `define((require, exports, module) => {
  ${code}
  ;return exports;})`

define(['babel'], babel => ({
  load: (name, req, onload, config) => {
    const path = req.toUrl(name) + '.jsx'
    fetch(path)
      .then(callMethod('text'))
      .then(transform(babel))
      .then(format)
      .then(onload.fromText.bind(onload))
      .catch(onload.error.bind(onload))
  }
}))

})()

参考文献

RequireJS使い方メモ
 GitHub - icecreamliker/requirejs-babel-plugin: A better RequireJS plugin for babel
RequireJS + babel-standalone + JSX