If you use a client-side js framework like JQuery or Prototype, the cross browser compatibility isn't an issue since it's taken care of for you.

The user agent it reports as is irrelevant - it's not a full http client. You can use other ruby tools to fetch a page if you want, and then feed harmony the document.

As others pointed out, testing is probably the main use case, but it offers other obvious possibilities; for instance, talkerapp.com needed a way to easily validate js syntax for their plugins.