I can see how this could be extremely useful, but it does strike me as yet another source for unique bugs in your JS-manipulating-DOM code, and it's potentially even harder to find out why they're happening.
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.
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?