I'm tempted to believe that companies that prefer long term employment are the ones that need to train employees for their specific needs. Companies that make use of transferable skills instead increase their chances of a new hire quickly picking up where the previous employee left. e.g. a web dev, in a FOSS shop that uses well known best practices.
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.