Hashbangs for state representation are just a tool, which can be used to make things better or worse for the users.

Where content is the main purpose ("websites"), they are overkill at best.

They can be useful though in functionality-first applications ("apps"), where the interface can be costly to build (too slow if you reload the whole "page" on each state change). Ideally, the app differentiates between fundamental states, represented in URLs, and transient states, represented in hashbangs.

Anyway, this is good news for Twitter, if they are really going through with it.