The question is does s.async = true have a use for dynamically inserted scripts, or are these loaded asynchronously already. The answer is they aren’t loaded asynchronously in all browsers, as explained here (thanks to Markus Olsson for the link)
script-inserted scripts execute asynchronously in IE and WebKit, but synchronously in Opera and pre-4.0 Firefox. In Firefox 4.0, the async DOM property defaults to true for script-created scripts, so the default behavior matches the behavior of IE and WebKit.
In browsers that support async but don’t already default to asynchronous loading (for example, Firefox 3.6), async = true makes a difference.
(The above link confirms that async is supported in Gecko 1.9.2, the layout engine used by Firefox 3.6)