While we don't use the Backbone.js framework, we have faced the same dilemma a while ago. We also ditched the hash/hash bang URL's in favor of the way nicer HTML5 History API.
Used correctly, URLs are a great thing. Web development is centered around them: Cool URLs don’t change, URLs as UI, RESTful HTTP interfaces, hackability and so on. The concept of HTTP URLs dates back to 1994.
Good URL design is, in my eyes, an essential part of a web application or even website's design.
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