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.

Nothing is irrelevant for a testing platform. For instance, how does it handle a canvas tag? It'd be useful to be able to dump out frames as images. A user agent string could be used by javascript to do any number of things, so I would hope it's modifiable.

As to the libraries to make things cross-browser compatible, knowing which browser it tries to emulate would be essential to knowing which lines of code would be executed through this tool. If you can't test the IE-specific code, what's the use if you're developing for IE?

I think it simulates a perfect browser. But that's fine. I see unit testing as the main use case for this library. If you want to test browser compatibility, then just simulate their quirks by mocking and stubbing the relevant APIs.